Diffusion effects on geminate charge recombination are investigated for intermolecular electron transfer system. Analytical and numerical calculations are presented for reaction-diffusion equations. In some limiting cases of the equations, a condition is established where the population of geminate ion pairs decreases exponentially by charge recombination. This condition is that the ion pairs are produced inside the zone of recombination and charge recombination takes place before ion pairs diffuse into separated ions. Numerical calculations are made in realistic cases where reorganization energy in the expression of the rate constant depends on the reactant distance. As a result, we found that the population of ion pairs has both components of exponential decrease and a root inverse tail (t(-1/2)). The component of exponential decrease is large when the recombination rate is fast.