We study the time dependence of donor-acceptor forward and back electron transfer in solids with restricted geometries in binary approximation. We focus on the time dependence of the donor cation state probability, P(t) within spherical and cylindrical volumes. Deviations from the known Forster type behavior are observed due to the geometrical restrictions. The effects of the finite volumes are pronounced when the volume dimensions are comparable to the critical radius for charge transfer. The space restriction slows down the charge separation. The results are good in the limit of low concentrations of accepters.