The thermal decomposition of ammonium copper(II) sulphate hexahydrate has been studied at temperatures between 339 and 393 K over a range of water vapour pressures. The compound loses four molecules of water of crystallisation in this temperature range, showing Avrami-Erofeyev kinetics at low temperature, changing to contracting-disc kinetics above 350 K. The Arrhenius parameters for the Avrami-Erofeyev region are E = 170 kJ mol(-1) and A = 2.1 x 10(21) s(-1), while the contracting-disc kinetics show E = 82 kJ mol(-1) and A = 2.2 x 10(8) s(-1). There is no observable effect from variation in water vapour pressure. Microscopy has been used to clarify the kinetics.