We describe a dehydration study of ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) organic composite membranes. This study gives an original description of the state of water in polymer composite membranes by differential thermal analysis (DTA). Usually three types of water are present in polymer membranes described by spectroscopic studies : first shell of water hydrate (type 1 water), second shell of disturbed liquid like water and liquid Like water (type 2 and 3 water). These experiments cannot permit to clearly distinguish these different types of water but only two. Ours results show that strongly bounded water, corresponding to the type 1 water, has a high departure temperature. Types 2 and 3 water cannot be differentiated and their departure occurs around the boiling point temperature of free water.