According to good old Wikipedia, the melting point of PET is 250*C (482*F). I can see the really cheap, flexible plastic water bottles deforming enough to leak if you put boiling water in them though. Some of them leak anyways once you open them. Some of the heaviest plastic bottles I've seen are the Smartwater bottles. I doubt that they would deform enough to leak. Might be time for an experiment smile The Smartwater bottles can also be found with a flip top which can also be used to back flush Sawyer water filters (if you use a Sawyer and like multi-use equipment).