The mechanism of the thermal decomposition of potassium permanganate crystals is critically reappraised through new microscopic and kinetic observations which are discussed with due consideration of the many and extensive literature reports that relate to this solid state reaction. We identify, from electron micrographs, a crystalline product of unusual texture that is formed early in the reaction. The topography of the residual products of the completely decomposed salt was, however, irregular and no significant textural features were identified. From consideration of the published reports relating to reaction stoichiometry and the chemistry of related reactions, we conclude that the solid products contain material that is inhomogeneous, poorly crystallized and possibly non-stoichiometric. The decomposition kinetics are shown to be satisfactorily described by the Avrami-Erofe’ev equation (n = 2) based on a nucleation-and-growth reaction model. We conclude that permanganate ions decompose in the poorly crystallized zone at the advancing KMnO4/product interface by an electron transfer process, in agreement with a previously published mechanism.