Fluorescence quenching by reversible excimer formation is studied on the assumption that excimer formation and dissociation can be modelled as entering and leaving the attractive region of an monomer excited-monomer interaction potential by diffusion. To get some general insight in the kinetic consequences of such a type of modelling, the simple case of an attractive square-well potential is investigated. It is shown that three different kinetic regimes have to be distinguished: Two "reversible" ones in case of slow excimer radiative decay, in which the quenching kinetics can be formulated by Markovian or non-Markovian rate equations with both excimer formation and excimer dissociation terms, and an effectively "irreversible" regime if the excimer radiative decay is too rapid to allow the excimer equilibration. In the latter case a dissociation coefficient can no longer be defined and the quenching kinetics can only be predicted on the basis of generalized rate equations of a net-excimer-formation type. It is shown how the quenching constant formula must be generalized to be applicable in all kinetic situations. (C) 2004 American Institute of Physics.