We report the first direct observation of power-law relaxation to equilibrium of the diffusion influenced reversible reaction, AB reversible arrowA+B (B>>A). Our experimental findings confirm the predicted t(-3/2) decay law relaxation of AB population a long time after the photoinitiation of the reaction. This t(-3/2) relaxation of the excess-over-equilibrium population is similar to that found in diffusion influenced geminate recombination reactions.