AbstractPhotochemical and thermal dissociation of labile molecules in the solid state or dissolved in solid polymer matrix affords reaction products which differ in composition and relative amounts from those obtained in solution. These differences are interpreted on the basis of limited rotation and diffusion possibilities. When the dissociation produces reactive species which can recombine thermally or photochemically, the kinetics do not follow a bimolecular reaction mechanism, as in solution experiments. On the contrary, they show a stepwise process, that has been described by Waite, Lebedev, and Butyagin. As a first step, rapid “cage recombination” occurs, followed by a second very slow step corresponding to the diffusion‐controlled recombination of isolated species (radicals or monomers). These stepwise reaction kinetics are illustrated by three different dissociation–recombination equilibria.a.thermaldissociation of benzpinacol and its derivatives and radical recombination.b.photochemicaldissociation of hexaphenyl‐1.2′‐bisimidazole into triphenyl‐imidazolyl radicals and their recombination.c.photochemicaldissociation of substituted anthracene‐ and (benz)acridizinium dimers andphotochemicalrecombination of monomers In (b), radical pairs and isolated radicals can easily be distinguished by ESR. On the basis of Waite's rate equations, the diffusion coefficient of the radical and the “cage” dimension can be calculated. The cage dimensions agree with these determined by Butyagin for radical decay on γ‐irradiated polymers. Experiments at different temperatures (below and aboveTg) stress the influence of the segmental mobility of the polymers, as well as of the molar volume