A new method for evaluating the rate constants of chemical reactions has been developed based on the differential photoconversion efficiencies of photochromic FRET acceptors incorporated into molecules on both sides of a chemical equilibrium. In the application of Photochromic Relaxation Kinetics (pcRelKin) to a bimolecular binding reaction, one of the two interacting molecules bears a FRET donor and the second a photochromic acceptor. The close proximity required for FRET is achieved only in the complex. After photoconversion, the system exists in a thermodynamically asymmetric state, which relaxes according to first order kinetics. PcRelKin offers unique advantages over classical relaxation kinetics.