Structural, thermodynamic and phase properties in the Na2S-H2O system for application in a chemical heat pump have been investigated using XRD, TG/DTA and melting point and vapour pressure determinations. Apart from the known crystalline phases Na2S.9H(2)O, Na2S.5H(2)O and Na2S a new phase Na2S.2H(2)O has been proven to exist. Na2S.1/2H(2)O is not a phase but a 3:1 mixture of Na2S and Na2S.2H(2)O, presumably stabilised by very slow dehydration kinetics. The vapour pressure-temperature equilibria of the sodium sulphide hydrates have been determined and a consistent set of thermodynamic functions for these compounds has been derived. XRD measurements indicate the topotactic character of the transitions between the hydration states.