Spectrophotometric and ESR measurements have been made in a study of photodecomposition and thermodecomposition reactions of spiropyrans (SPs) of the indoline and oxaindan series of solutions and in the crystalline state, at various temperatures. From an analysis of the experimental data, involving certain concepts developed previously regarding the mechanisms of photochemical reactions of SPs, it has been concluded that the most probable reactions of photodecomposition and thermodecomposition of SPs are α-rupture of a bond and the formation of a chemically active biradical, as well as irreversible reactions of SP ion radicals that arise as a consequence of phototransfer and dark transfer of electrons in associates of various species of the SP.