Dehydrations of α·NiSO4·6H2O single crystals cleaved parallel to (001) have been performed both at constant and linearly increasing temperature. The thermal curves recorded are found to change remarkably in shape by changing the rate of temperature increase (scan speed). A number of unforeseen events, like fluid formation and bubble swelling, taking place in the crystal under investigation were visually observed and photographically recorded by coupling a Wild microscope to a Perkin-Elmer DSC-1b apparatus. An interpretative hypothesis is proposed based on the initial formation of a dehydrated layer on the crystal surfaces. The properties (perfection, continuity and permeability to the gaseous product) of such a layer are one of the parameters governing the evolution of the system. Other parameters are the rate of product crystallization, the rate of inner gaseous product generation and their variations with temperature.