Relaxation kinetics of a pseudo-first-order reversible association-dissociation reaction system perturbed by photolysis is discussed by simple probabilistic arguments. Response to the photolytic perturbation is expressed in terms of the response to the thermal perturbation. New power law behavior in the asymptotic relaxation of the concentration deviation is obtained with exponents 23͞2 and 25͞2 rather than 21͞2 and 23͞2 in one and three dimensions, respectively. [S0031-9007(97)04516-X]