AbstractThe first part of this paper constitutes a summary account of reactive phase formation as seen from a purely kinetic point of view. In the longer second part a more detailed analysis of phase formation based on microscopic, and calorimetric observations is presented. It is shown that these phenomena also tend to be dominated by kinetic factors. This explains why purely kinetic considerations (which should always prevail in any case at some point) usually provide a fairly accurate account of the growth of new chemical phases forming on a solid substrate, regardless of the physical state (solid, liquid or gas) of the second reactant.