Modern laser-chemistry techniques allow the preparation of initially correlated pairs as precursors to electron or atom exchange in solution. We obtain a rigorous analytic solution for the two coupled 'backward' diffusion equations describing the survival and reaction probabilities. Investigating its long-time asymptotic behavior reveals a transition from monotonic to non-monotonic approach to equilibrium, usually unexpected for elementary single-step reactions. We suggest the synthesis of 'detachable bifunctional compounds', that may undergo peculiar kinetics as described herein. (C) 2003 Elsevier Science B.V. All rights reserved.