The importance of the reaction volume Delta V-cc of contact complex formation for the interpretation of reaction and activation volumes is quantitatively investigated on the basis of literature data. The deactivation of singlet oxygen O-2((1)Delta(g)) via electronic to vibrational energy transfer is used as a probe for the study of the influence of the liquid structure on rate constants and activation volumes, Delta V-cc reaches values up to about -10 cm(3) mol(-1) for the formation of contact complexes of O-2 with polyatomic quenchers in alkanes. The same volume effect but with opposite sign contributes to the reaction volume of the fragmentation of diphenylcyclopropenone, in which the diatomic CO dissociates apart from a polyatomic fragment. Reaction volumes of excimer and exciplex formation are exemplarily reinterpreted on the basis of these results. The consideration of Delta V-cc leads to very different mechanistic conclusions in comparison to earlier interpretations.