We report a study of the thermal decomposition of rubidium permanganate. Isothermal kinetic measurements of the evolved oxygen produced showed a sigmoid fractional reaction alpha - time relation with a rate maximum at alpha almost-equal-to 0.65. Data fit the power law alpha1/3 = kt in the range 0.04 < alpha < 0.6-0.7; this is an unusually extended acceleratory phase. Subsequently, decomposition is relatively rapid and is completed in a short deceleratory process. The results are not satisfactorily expressed by rate equations characteristic of solid-state reactions and it is concluded that local and/or temporary melting participates in controlling the kinetic behaviour. Some fusion of the product is consistent with the textural changes that accompany salt decomposition, as revealed by scanning electron microscopy. We conclude that, with the participation of local and perhaps temporary melting, the overall decomposition contains features of both homogeneous and heterogeneous reaction mechanisms.