Measurements of the rate of decomposition of solid potassium azide in hard vacuum and in a controlled vapour pressure of potassium are reported. Some observations of F-centre formation and decay allow a model for the trapped exciton to be formulated. This concept of a coloration complex is consistent with the previous theory postulated for the photo-chemical decomposition, with the F-centre bleaching rates and with the ineffectiveness of pre-irradiation on subsequent thermal decomposition. The thermal decomposition consists of an initial decay process, associated with anomalous groups of surface defects, followed by a linear process corresponding to an interface reaction. Nucleus formation and growth are absent because of the volatility of the metallic product. The decomposition is catalyzed by the presence of a constant vapour pressure of potassium, the increase in rate being due solely to a reduction in the activation energy of the reaction by the provision of electron traps other than vacant anion sites.