We study reaction-diffusion systems which involve processes that occur on different time scales. In particular, we apply a multiscale analysis to obtain a reduced description of the slow dynamics. Under certain assumptions this reduction yields a new set of reaction-diffusion equations with rescaled diffusion coefficients. We analyze the Selkov model [E. E. Selkov, Eur. J. Biochem. 4, 79 (1968)] and the ferrocyanide-iodide-sulfite reaction [E. C. Edblom et al., J. Am. Chem. Soc. 108, 2826 (1986)] to determine whether the rescaling in this case may account for the difference of diffusivities that the formation of certain types of patterns requires.