A qualitative analysis is presented of the interrelationships between phase transformations and organic chemical reactivity in the solid state, taking into consideration general thermochemical relationships and the thermodynamics of heterophase equilibrium. Two cases, where isomerization reactions depend on the solid-state solubility of the reactant and product, are considered and show that the formation of a new phase can influence both the reaction yield and rate. For example, it is shown that crystallization of a new phase from a crystalline or amorphous solid solution can supply the thermodynamic driving force for chemical transformation. Formation of a new phase may influence solid-state kinetics depending on the solubility of a reactant in the new phase and the relative rates of chemical transformation and formation of the new phase. It is further shown that even for simple monomolecular reactions, kinetic curves for the overall process can consist of up to five parts, depending on the type of phase diagram involved. These principles have been applied to some examples of solid-state isomerization in a way that allows the choice of a proper kinetic scheme and an explanation of the direction and maximum yield observed for a particular reaction. approaches include the 'topochemical principles' of Cohen and Schmidt,' reaction cavity by Cohen,2 the role of local stress by McBride et ~1 . ~ and in large part the theory developed by Luty and E~khardt.~ Such studies allow one to consider the role of crystal structure and, in particular, the significance of defects originally present or generated as a result of the reaction.