Salomon Soft Flask 500ml/17oz Ultra 42 Review

Overview

The Salomon Soft Flask 500ml/17oz Ultra 42 is a trail-running-first hydration flask designed to ride in the front pockets of a running vest or a waist belt. The “Ultra” designation sets it apart from Salomon’s Speed model: where the Speed has a rigid thermomoulded base that lets it stand upright, the Ultra is soft from cap to base — fully collapsible, lighter, and shaped with an anatomical tilt so it sits flush against your chest rather than jutting out. It’s aimed squarely at trail runners and ultramarathon racers who want a no-fuss, low-weight flask they can drain and forget about until the next aid station.

Worth noting upfront: Salomon soft flasks are manufactured by HydraPak. That matters when it comes to warranty claims and understanding how the product compares to other HydraPak-built flasks on the market.

Key Specs

Spec	Value
Weight	38 g (1.34 oz)
Capacity	500 ml (17 oz)
Opening Diameter	42 mm
Dimensions (L × W × D)	30 × 9 × 5 cm
Material	90% TPU, 10% Polyoxymethylene (POM)
BPA-Free	Yes
Valve Type	High-flow bite valve
Approximate Price	~$25 USD
Comparison	See how Soft Flask 500ml/17oz Ultra 42 compares to similar gear

Performance

Fit in vest pockets

The defining characteristic of the Ultra is its geometry. It has an anatomical tilted design that fits comfortably close to your body, and the conical shape lets it effortlessly slide into place with no uncomfortable pressure points, while the tilt is designed to give easier and faster access to hydration while running. The sloped, soft base is the real practical win here — it makes it easier to glide the flask into your vest pocket.

That said, the Salomon Soft Flask is 11.8 inches tall, which is notably taller than shorter, stubbier alternatives. It works best with the Salomon ADV Skin vest’s deep front pockets, and also performs well in vests like the La Sportiva Ultra Trail Vest that share a similar deep-pocket design. If you’re running a shallower vest pocket, the flask may sit high and bob around — some users report this as a fit issue outside of the Salomon ecosystem.

Filling and drinking

The wide-mouth design includes extra silicone reinforcement for fast refills.

In practice, that 42mm opening is wide enough to accept ice cubes on hot days and to get electrolyte tablets or powder in without dusting yourself — a real-world frustration with older, narrower-necked flasks.

The high-flow valve releases fluid faster to keep you hydrated without any fuss.

The BPA-free materials and collapsible design mean the flask shrinks as you go, eliminating water sloshing and keeping you focused on the run.

Multiple users confirm the valve performs well under duress —

the bite valve is easy to operate and allows for high flow, and some users report no leaking even when the flask is pressed firmly against them in a belt.

Cleaning

The wide 42mm opening is a genuine upgrade for cleaning. Its advantages include easier cleaning compared to narrower-necked alternatives. The one housekeeping note: it can be a bit difficult to get the rubber bite valve off for cleaning at first, but users consistently report getting the hang of it quickly.

Taste and off-gassing

TPU offers elasticity, transparency, resistance to liquid penetration, and is described as taste-free and non-toxic.

Most users confirm there’s minimal or no lingering plastic taste.

There may be a faint plastic taste at first, but it goes away after a good cleaning and first use.

Durability

This is the area that splits opinions. The majority of long-term users are positive: the material holds up surprisingly well even under heavy use, and mold growth has not been a widespread concern even when water has been left inside. One user on the UKC forum notes no problems with leaks after 3 to 4 years of use.

However, there’s a meaningful minority of users who’ve had problems. Some have had to replace flasks multiple times in a year due to tiny holes developing in the body — causing leaking when full and under pressure inside a vest. Separately, at least one user reports the internal valve breaking while drinking on only the second use, which they flagged as a potential choking hazard. These failures don’t appear to be the norm, but they’re frequent enough in user reviews to flag. If yours does fail, the warranty is handled through HydraPak directly.

Ecosystem and compatibility

The 42mm opening is compatible with the Salomon XA and Katadyn BeFree water filtration lids — meaning you can turn this flask into a portable squeeze filter just by swapping the cap.

That’s a meaningful bonus for ultrarunners on courses with natural water sources, or backpackers who want a lightweight filtration option.

Pros & Cons

Pros

• At 38g, it’s one of the lightest 500ml flasks in this format
• Anatomical tilt and conical shape slide cleanly into deep-pocket vests
• 42mm wide mouth accepts ice, powder, and tabs without fumbling
• Shrinks as you drink — genuine reduction in bounce and slosh
• Compatible with Salomon XA and Katadyn BeFree filter caps
• BPA-free TPU with minimal taste off-gassing after initial rinse
• Strong long-term durability track record for most users

Cons

• 30cm tall — can be too long for vest pockets outside the Salomon ADV Skin family
• No rigid base: won’t stand upright for filling at a tap or aid station
• A subset of users report body leaks and valve failures — quality control is inconsistent on some batches
• Bite valve requires mild effort to remove for thorough cleaning
• Not insulated — water temperature tracks ambient conditions quickly
• No flip-lock cap: the bite valve can spray if bumped in a belt

Who Should Buy This

This flask is the natural choice if you’re already running a Salomon ADV Skin vest or another vest with tall, narrow front pockets. It’s also the right call if you want 42mm filter cap compatibility for courses with water sources — the Salomon XA or Katadyn BeFree cap swaps on without hassle. It’s less ideal if you’re pairing it with a shorter-pocketed vest, prefer the stability of a rigid base at aid stations, or want a locking cap for belt use. In those cases, look at the Salomon Speed 42 (rigid base) or the HydraPak Speed Flask with its flip-lock lid.

Verdict

The Ultra 42 does its core job well: it’s light, fits tightly against your chest when the vest is right, fills fast, and drinks fast. The 42mm opening alone makes it more practical than many older flask designs, and filter cap compatibility is a quietly useful bonus. The durability record is good for most users, but the occasional reports of body pinhole leaks and valve failures are a real consideration — buy a spare if a race depends on it. At around $25, it’s not cheap for a plastic bag with a valve, but the HydraPak construction and ecosystem compatibility justify the premium over no-name alternatives.