The kinetics and mechanism of the thermal decomposition of sodium sulphide pentahydrate to an essentially anhydrous form was investigated, in the pressure range 3–20 Torr, using thermogravimetry and X-ray powder diffractometry under controlled temperature and water vapour pressure. A two-step dehydration reaction pathway involving an intermediate dihydrate phase was observed: Na₂S·5H₂O → Na₂S·2H₂O + 3H₂O (g); Na₂S·2H₂O → Na₂S + 2H₂O (g). The latter reaction is shown to be relatively more inhibited than the first. The intermediate phase was studied by means of simultaneous thermogravimetry and X-ray powder diffraction methods. A structural transformation is proposed in terms of a topotactic process, which readily accounts for the readiness with which the first reaction takes place and which is also consistent with the overall structural transformation from the pentahydrate to the anhydrous product.