GSI Outdoors Microlite 350 Flip review: an excellent light flask for keeping drinks warm or cold

The GSI Outdoors Microlite 350 Flip’s specially designed flip top lets you drink hot or cold liquids straight from the flask

GSI Outdoors Microlite 350 Flip
(Image: © Jonathan Manning)

Advnture Verdict

Whether you’re looking for a belly-warming hot brew or a refreshing cold drink to quench a hard-earned hot thirst, the GSI Outdoors Microlite 350 Flip will do the business. An excellent flask for taking out on trail adventures of all kinds.


  • +

    Drink straight from the flask

  • +

    Light, slender design


  • -

    Separate mug required if drink is too hot to swallow straight from the flask

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First impressions

The slim, stylish GSI Outdoors Microlite 350 Flip applies the spout technology of a water bottle to the science of keeping drinks kettle hot or ice-cube cold. The flip-top lid and pouring spout lets you swig straight from the flask, which is ideal unless your drink is too hot to swallow (seriously, be careful!)

The ultra-thin walls (just 2mm thick) of the flask lend it the slenderness of a supermodel, but it remains capable of swallowing a surprisingly large volume of fluid given its dimensions – GSI reckons it can carry 25% more liquid than traditional vacuum bottles of the same size.

As for its insulation, GSI says the GSI Outdoors Microlite 350 Flip will keep hot drinks hot for eight hours and keep cold drinks chilled for as long as 16 hours. So we put it to the test.


RRP: £28 (UK) / $25 (US)
Volume: 350ml /12fl oz
Other available volumes: 500ml / 17fl oz; 720ml / 24fl oz
Weight (empty): 200g / 7oz
Height: 18cm / 7.1in
Lid-opening style: Flip top
Colors: White / Black / Fuchsia / Red / Sky Blue

In the field

Despite not having an orthodox screw-in stopper, the GSI Outdoors Microlite 350 Flip flask performed impressively when we put it through our heat retention test; boiling water had cooled to 68°C after four hours.

What’s more, despite our best efforts to shake, tip and force a leak of any type, the lid closure proved drier than the Sahara in summer, swiftly engendering complete confidence in the GSI.

The push-button, flip-top cap allows one-handed access, which is convenient if you have got a map, GPS, camera or walking pole in your other hand. This makes a great flask for taking hot or cold drinks on various outdoor escapades. Alternatively, if you want a better solution to a thermal mug for your commutes, look no further.

Jonathan Manning

After spending a decade as editor of Country Walking, the UK’s biggest-selling walking magazine, Jonathan moved to edit Outdoor Fitness magazine, adding adrenaline to his adventures and expeditions. He has hiked stages or completed all of the UK's national trails, but was once overtaken by three Smurfs, a cross-dressing Little Bo Peep, and a pair of Teletubbies on an ascent of Snowdon. (Turns out they were soldiers on a fundraising mission.)