- The reversible A↔B reaction-diffusion process, where species A and B are initially mixed and diffuse with different diffusion coefficients, is usually simulated in the framework of the quasiequilibrium approximation. In this paper the reaction rate of such process is investigated. It is shown that despite the fact that a reaction-diffusion process is considered as the quasiequilibrium process the contribution of the reaction in changes of the species concentration is comparable with the diffusion contribution. Moreover, the ratio of the reaction to diffusion contributions is independent of time and coordinate. Using published experimental data the reaction rate and its profile are calculated.