The material used in the standard Sea to Summit Compression Sacks is polyurethane coated 210D Nylon. (Nylon by itself is not waterproof, and needs to be coated to make it so). However, not only do these bags *not* have a roll-top closure (like Sea to Summit eVent Compression Dry Sacks), they are not seam-taped either: so water can make its way through the sewn seams. (This is true of all brands of compression sacks, not just Sea to Summit products) - If you need a basic, robust compression sack, these bags will do the job (and the lid is designed so it won't flip over and tangle, which is a nice feature) - if you want the same degree of compression, but would like to save weight, check out the Sea to Summit UltraSil Compression Sacks (lightweight, non flip lid, but still without taped seams) - if you want compression *and* you need to keep your gear dry, the Sea to Summit eVent Compression Sacks is the gear of choice And if you'd like any futher explanation (honest answers, not marketing spin) shoot me an email at barry@seatosummit.com Cheers! Barry Robertson Sea to Summit Customer Relations Manager

I second the recommendation for the eVent bags. I have a few for my sleeping bag and clothes and they work quite nicely. Only about $10 more than these and worth every penny: http://www.backcountry.com/sea-to-summit-event-compression-dry-sack

Eldad, Angus is correct in the true defintion of waterproof, but the material is nylon/cordura thus the actual material is waterproof to an extent, but the top is not closed like the eVent dry sack made by Sea to Summit. So if you were to dunk this or let it sit out in a storm you might have some issues. If this is in you pack while its raining, no worries your stuff will stay dry.

No.