Fast reversible pseudo-unimolecular reactions do not approach equilibr
ium in the simple exponential fashion predicted by chemical rate equat
ions. Many body effects in bimolecular reactions have been revealed in
an extensive series of Brownian simulations, showing up to three dist
inct phases in the approach to equilibrium. Recent theoretical develop
ments are compared with the simulation as well as with experimental da
ta concerning the pH effect in excited-state proton transfer to solven
t.