Although the dehydration of lithium sulfate monohydrate in vacuum (almost-equal-to 10(-4) Torr) has a simple appearance, the kinetics is not so simple as the reaction mechanism, which can be expressed by a single function model F(alpha). The kinetics is represented by the combination of A2 and R2 functions. The initial acceleratory stage described by the A2 equation seems to be related to the nucleation and nuclei growth process controlled by the diffusion of dissociated water molecules. When the two-dimensional phase boundary is formed around the specimen by the growing nuclei, the dehydration seems to be controlled by the advance of the interface and to have the characteristics of the R2 model function. The values of activation energies and pre-exponential factors were 22.1 kJ mol-1 and 1.45 s-1 for the A2 process, and 29.3 kJ mol-1 and 12.1 s-1 for the R2 process, respectively.