Two typical dehydration pathways of alpha-NiSO4.6H(2)O were identified through simultaneous measurements of TG-DSC or TG-DTA under various sample and measuring conditions, complemented by powder X-ray diffractometry, FT-IR spectroscopy, and direct observation of the intermediate compounds. One is a quantitative isothermal dehydration to the dihydrate observed at 350-370 K in a flow of N-2 with the sample mass less than 15 mg. For the isothermal dehydration from the hexahydrate to the dihydrate, it was shown that the process consists of surface nucleation of the dihydrate and growth of the nuclei, followed by the advancement of the resultant reaction interface inward. The first process on the surface and succeeding reactions are characterized kinetically as obeying the first order and phase boundary controlled laws, respectively. The other pathway is obtained for the nonisothermal reactions at a higher partial pressure of the self-generated water vapor. The process is composed of three distinguished dehydration steps to the anhydride via roughly the tetra- and monohydrates. The first step of reaction is surface nucleation of the tetrahydrate, being different from the isothermal reaction, followed by formation of the specific surface produce layer on the dihydrate, which influences the advancement of the reaction interface of the hexahydrate-tetrahydrate and diffusion of the evolved water vapor. The second step includes crack formation which acts as channels for diffusion of the water vapor evolved in the subsequent internal reactions. The reaction behavior of the third step varies with the heating rate, which is related to variation in the morphology and geometry of the intermediate monohydrate. Significance of the interpretation of the kinetics and mechanism of the solid-state decompositions is exemplified by a comparative study of the two characteristic dehydration behaviors under isothermal and nonisothermal conditions.