The dependence of the growth rate vg of nuclei during CuSO 4·5H 2O dehydration upon water vapor pressure PH 2O has been investigated. At small PH 2O (less than 1 Torr at 50°C), vg falls rapidly with decreasing PH 2O , and simultaneously vg anisotropy disappears. At PH 2O = 1-9 Torr, vg for elliptic and round nuclei is constant and independent of PH 2O . The influence of initially keeping the samples in water vapor upon the kinetics of the following dehydration has been studied. Treatment by water vapor leads to slowing of dehydration and increasing its degree of transformation. On the basis of data obtained a kinetic interpretation of the Topley-Smith effect has been proposed.