Computer simulation of the layer-by-layer distribution of temperature and dehydration rate J in powdered Li2SO4 . H2O monohydrate as functions of the partial pressure P-w of water vapor reveals a substantial self-cooling of the sample. The anomalous course of the J=f(P-w) curve with increasing P-w, which manifests itself in the appearance of a maximum in the curve (the Topley-Smith effect), originates from the competition of the depressing action of H2O vapor on the dehydration rate with increasing heat transfer from the furnace to the sample. The model accounts for the main features of the Topley-Smith effect, namely, the falloff of decomposition rate for low P-w and the minimum in the J=f(P-w) curve, the variation of the value of P-w corresponding to the maximum of the curve within two orders of magnitude for different hydrates, and the enhancement of the effect with increasing decomposition temperature and decreasing grain size of the powder sample.