Thermal decompositions of precompressed and uncompressed samples of KBrO3, KMnO4 and NH4ClO4 have been studied by thermogravimetry under isothermal conditions in static air. The rate of decomposition increased with increase in the applied pressure in the case of the last two solids, but the rate dramatically decreased in the case of KBrO3. The results indicate that precompression generally sensitizes electron transfer reaction as a result of an increase in the dislocation density, whereas it desensitizes diffusion-controlled reactions as a result of the densification of solid matrix. The method of precompression is suggested as a quick and simple tool for testing whether a decomposition reaction is diffusion controlled or not.