We report a first microscopic simulation for the diffusional kinetics of a reversible excited-state reaction, A+B<----> AB, where A and AB have different lifetimes and the B-particles are in excess. When the excited species equilibrate fast compared with the excited-state lifetimes, one obtains the pre-equilibrium approximation. The full time-dependence in this case is approximated by a shifted infinite-lifetime expression, and this allows us to derive an analytic expression for the asymptotic exponent. Multiplying this exponential is a t(-3/2) term. When the excited-state decay of AB is fast, we obtain the quasistationary approximation. Quantitative comparison between various theories and simulation is presented.