Mortar expansion data have been fitted to various kinetic models. The expansion tests utilized several aggregates (Pyrex glass, Vycor glass, calcined flint, Spratt's aggregate, and Beltane opal) and cements with various alkali contents. A very good fit was observed using a model developed to describe autocatalytic chemical reactions, and a fairly good fit was observed using a model for topochemical reactions. Microstructural studies showed that the reaction of Pyrex and Vycor involved highly localized attack of the glass, while reaction of calcined flint was less localized and appeared to be associated with preexisting internal fractures. For all three aggregates, the mortar expansion appeared to be associated with expansion of individual aggregate particles.