We report a microscopic simulation for the reversible A + Breversible arrowC + D reaction in its pseudo-unimolecular limit. The simulations are used to assess the validity of various approximate theories for the time dependence of the approach to equilibrium, up to its long-time asymptotic behavior. To available theories we have added a new theory, the generalized multiparticle kernel 1 (GMPK1) theory. It is shown to be in very good agreement with our simulations than all other theories. The asymptotic behavior of both simulations and GMPK1 agrees with that derived by Gopich and Szabo [Chem. Phys. 284, 91 (2002)]. (C) 2003 American Institute of Physics.