The phase diagram of the N-methylmorpholine-N-oxide-H2O mixtures from 0 to 100% has been determined. Three crystalline hydrates have been identified, the already known monohydrate, a dihydrate and a hydrate composed of 8 water molecules per NMMO. The melting temperature of the 8H(2)O-NMMO hydrate is -47degreesC with a melting enthalpy of about 80 J/g. The region between 25 and 55% of water does not show any crystallisation, but a glass transition around - 60 to - 100degrees C.