Total rate constants of decay (k_t) as a function of temperature from −45 to 65°C for the compounds 1 and 2 in AN and TFE and 3 and 4 in AN have been determined by fluorescence lifetime measurements. The data have been fit to an equation that assumes that the rate constants of fluorescence (k_f) and intersystem crossing (k_isc) are temperature independent, that k_ic = 0 and that the rate constant of reaction (k_r) is activated according to the Arrhenius expression. For compounds 1–3, values of k_f and k_isc were found to be independent of solvent for any given compound, but k_r was consistently greater in TFE than AN. For the anisoles 4, the temperature effect was very small, indicating that k_r did not compete with k_f k_isc and suggesting that an activated intersystem crossing was the dominant temperature-dependent process. The k_r, A and E_a values obtained for compounds 1–3 were rationalized in terms of their known photochemistry, phototransposition reactions in AN and photoadditions in TFE. The critical reactive intermediate in all cases is a bicycle[3.1.0]hexenyl biradical/zwitterion that is formed in an activated process from S₁. This reactive intermediate returns to starting material faster than it rearranges, and therefore an activated internal conversion is a major pathway for deactivation of S₁.