I know in the Q&A it says these caps specifically do not work with S'ip bottles, but I decided to get one anyway and try it because it's easier for my pre-school age son to use these caps with the smaller bottles. I figured I would just use it for my own S'well bottle if it didn't work on his. It fit perfectly! He has a 10 oz S'ip by S'well bottle and no leaking, no issues at all. It's easy to remove the cap that comes with the S'ip bottle and replace it with this one. So in case there are any parents out there looking for the answer to this question-it DOES work! I would just caution you that it isn't a "throw in a backpack" type cap because if something hits it the wrong way it will click open and leak.