The thermal dehydration of hexahydrated nickel sulphate crystals grown by slow evaporation of aqueous solution maintained at 40 ± 1 ° C have been studied using dynamic and isothermal TG techniques. The dynamic TG measurements suggest the scheme of dehydration to be 1 mol, 2 mol, 2 mol and 1 mol at mean temperatures of 143, 180, 230 and 358 ° C, respectively. However, isothermal measurements suggest that the dehydration scheme is temperature-dependent. Many features of random nucleation and growth have been observed on the (001) face of the crystal under an optical microscope. The isothermal and dynamic TG measurements together with the optical microscopic study suggest that the dehydration of NiSO 4 · 6H 2O proceeds through random nucleation and growth. The proper form of F(α) could not be decided.