Diffusion‐controlled reaction kinetics of a molecular tracer was investigated in the miscible region of binary polymer mixtures. The reaction kinetics does not follow the Kohlrausch‐Williams‐Watts (KWW) mechanism and is controlled by polymer segmental free volumes in the context of classical free‐volume theory. Upon approaching the phase boundary, it was found that the reaction rates were affected by the concentration fluctuations with the wavelengths comparable to the size of the tracer in accord with the results obtained by small‐angle neutron‐scattering experiments.