The reactivity behaviour of porous catalyst particles subject to both internal mass concentration gradients as well as temperature gradients, in endothermic or exothermic reactions has been analysed. The analysis uses the exponential relationship between intrinsic reaction rate and temperature without approximations. Criteria for the existence of appreciable mass and heat diffusion effects, their quantitative appraisal, and their significance and importance in various types of actual catalytic solids are discussed. Regions of inherent instability of operation are shown to exist, and the conditions for their existence are demonstrated.