The kinetics of powder reactions has been revised to account for heat treatments with variable temperature. The driving force for diffusion controlled behaviour has been expressed in terms of a chemical potential difference across the product layer, which is usually temperature dependent. This was the basis to derive the relevant solutions for diffusion control with variable temperature. The effects of the rate of change in temperature and particle size were accounted for, and can be used to assess whether diffusion control or another controlling mechanism prevails. This test can be applied even for cases when some of the temperature dependent terms are unknown.