The possible role of melting in thermal reactions of initially solid reactants is not always explicitly considered in the interpretation of kinetic data and the formulation of reaction mechanisms. The present paper discusses reasons for the enhancement of reactivity on fusion and contrasts the rate characteristics of heterogeneous (solid state) with homogeneous reactions. The value of complementing kinetic measurements with analytical data and microscopic textural observations is discussed. The importance of this approach in elucidating meaningful reaction mechanisms is illustrated by considering features of selected kinetic studies of reactions in which it was demonstrated that there was the essential participation of perhaps local and temporary fusion or liquid intermediate formation during chemical changes that had initially appeared to proceed in the solid state. Literature examples are discussed in the text.