The free-energy gap law in back electron transfer reactions is shown to be affected by encounter diffusion in photogenerated ion pairs. The ion separation quantum yield increases when initial separation of ions is large compared to the radius of the reaction layer and decreases in the opposite situation when ions are created in contact but recombine outside. The effect changes the sign with free energy when passing from the normal region of recombination to the inverted one. As a result, the top of a bell-shaped curve representing the free-energy gap law is subjected to qualitative distortion, which is stronger when diffusion is slower.