We present an analytical theory of geminate charge recombination in donor-acceptor heterojunction systems based on the Smoluchowski equation. We show that the differential equation describing this problem is mathematically equivalent to that describing geminate recombination in a four-dimensional anisotropic system. By combining this result with already available analytical results for n-dimensional homogeneous systems, we predict that the survival probability of electron-hole pairs in heterojunction systems decays as t(-1) at long times. We prove this by analyzing the results of kinetic Monte Carlo simulations. We also explain clearly why the ultimate escape probability is increased by introducing the heterojunction. (C) 2012 Elsevier B.V. All rights reserved.