A step on the reaction path of some stepwise reactions in which the molecular entity C is already present in an encounter pair or encounter complex with A during the formation of B from A, e.g. In this mechanism the chemical species C may but does not necessarily assist the formation of B from A, which may itself be a bimolecular reaction with some other reagent. Pre-association is important when B is too short-lived to permit B and C to come together by diffusion.