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Can the Merrell Siren 4 Thermo Mid Zip Waterproof Boots back up to their funky look when it comes to performing on a British winter walk? We put them to the test to find out.

1lbs-3.04oz, 540g per boot
They look great, and keep you warm and dry all day on the trails. 
Zip opening may be redundant.

Sizes: UK 2.5 - 9

Buy Merrell Siren 4 Thermo Mid Zip Waterproof Boots: £180 on

TL;DR Review

A great looking winter hiking boot, the Merrell Siren 4 Thermo Mid Zip Waterproof Boots are a fantastic choice for those chilly winter walks.



Founded in 1981 by two Rossignol executives, Merrell is a go-to footwear brand for many an outdoor lover. Offering a wide range of products from technical winter hiking boots to lightweight trail running shoes, every active niche is catered for.

The Siren 4 Thermo Mid Zip Waterproof Boots slot neatly into the winter boot category, while staying stylish enough to not stick out if worn around town or in the pub at the end of a day out on the trails. 

Merrell Siren 4 Thermo Mid Zip Waterproof Boot review

Who are the Merrell Women's Siren 4 Thermo Mid Zip Waterproof Boots aimed at?

The Merrell Women's Siren 4 Thermo Mid Zip Waterproof Boots are a great choice for those who gravitate towards a leather boot and still want something that is going to perform technically, but that looks stylish and doesn’t get too bogged down in endless features. 

Merrell Siren 4 Thermo Mid Zip Waterproof Boots materials & their meaning:

First thing to mention: the outers of the Sirens are primarily made of full grain leather, so vegans steer clear. We are fans of leather boots here at Active Traveller and appreciate the style and performance of a leather outer. The Sirens also have a waterproof membrane for extra protection against water ingress, while also allowing the material to breathe and let moisture escape. We have no complaints when it comes to staying dry while tramping about the damp English countryside wearing the Sirens.

Moving further south, the Sirens feature a Vibram® Icetrek outsole which gives a sturdy, grippy tread surface even in icy conditions.

The waterproof and sturdy features are great, but the main selling point of this boot is the warmth. From the Solarcore® Aerogel insole which is designed to provide underfoot warmth, to the 200g of Primaloft® Gold Eco Series synthetic insulation inside the boot, Merrell take cold feet very seriously. And it seems to have paid off! We took the Sirens out for walks on frigid, frosty mornings as well as warmer, early spring days and found them great at keeping our feet toasty.

Merrell are also stepping up on sustainability with these boots with several features made from 100% recycled materials, such as the laces and webbing, fleece lining and mesh footbed cover. 

Merrell Siren 4 Thermo Mid Zip Waterproof Boot review

Style, fit and features:

The Merrell Siren 4 boots are very interesting stylistically. They are mostly a serious, leather, winter hiking boot, but their serious edges are softened by the addition of a knit upper, which gives it a funkier look. We like this addition, but it is a feature that isn’t waterproof, so if you’re out in a proper downpour, the stylish knit fabric might get a little soggy.

Comfort-wise, the Sirens have a FloatPro™ Foam midsole for those long trekking days, plus a recycled fleece lining which is nicely soft and a welcome addition. We found the fit to be fairly true to size when wearing with walking socks, plus the boot isn’t too narrow and fits us nicely across the ball of the foot.

One other feature we need to mention here is that the Siren 4s also have a heavy duty waterproof YKK zip in addition to fully-functional laces. Now, we like this addition, as it definitely makes getting the boots on and off a lot easier. However, we found that is isn’t a replacement for the laces. If the laces are done up tight for walking, the boot is too tight to the foot to remove by just undoing the zip. Both the zip and the laces need to be undone to get the foot free. This may be the intended use for the zip – as an extra feature in addition to the laces, rather than a replacement – but if this is the case, are both really needed?

Tester’s verdict:

Overall, we love the Merrell Siren 4 Thermo Mid Zip Waterproof Boots. The combination of waterproofing, insulation, comfort and a unique look makes them a great companion for any winter or chilly spring or autumn walk. 

Buy Merrell Siren 4 Thermo Mid Zip Waterproof Boots: £180 on

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