Abstract A series of complementary kinetic studies of Ca(OH) 2 dehydration showed that the reaction is deceleratory throughout and that isothermal yield-time data were satisfactorily described by the first order equation. Arrhenius parameters and relative reaction rates are sensitive to the reactant mass and its compaction/dispersal within the reaction zone. Rates of water evolution are significantly influenced by the pressure of water vapour prevailing, the local concentrations of water vapour within the reactant mass and the rates of intracrystalline and intercrystalline escape of water vapour from the reactant assemblage. The theoretical significance of the present observations are discussed in the context of electron microscope observations and the extensive literature already available concerned with this reaction. We formulate a reaction model that takes due account of previous proposals, for homogeneous or for heterogeneous-type mechanisms. We conclude that reaction advances inward from initial crystal boundaries but that water elimination is by diffusive loss from within an extended zone of maintained reaction crystal structure. Dehydration does not occur at a sharp reactant-product interface, unlike many solid state processes, and the kinetic characteristics are more deceleratory than the requirements of the contracting volume rate expression. Water loss is not closely followed by recrystallization; these steps are separated in space and time.