Abstract A theory for predicting the approach velocity of a vesicle diffusing toward another vesicle or plasmalemma membrane is presented. This theory takes account of the shift between retarded and nonretarded van der Waals forces of attraction, the electrostatic forces of repulsion, and a spatially varying diffusion coefficient that considers the changing viscous resistance as the intervening fluid gap narrows. Velocities are calculated as a function of gap distance by substituting the above-mentioned forces, which are found by differentiation of existing expressions for free energy of interaction, in a quasi-steady equation of motion. Calculations are presented which show that it is necessary to have a correct hydrodynamic description of the process in order to obtain accurate velocities. It is shown that a critical spacing dependent on cation concentration exists, where the velocity of approach becomes zero due to the presence of a secondary minimum. For a Ca 2+ concentration of 5 × 10 −2 M, a Na + concentration of 0.1 M, and a pH of 7.4, this distance is found to be less than 20 Å for a 700-Å phosphatidylserine vesicle approaching a plasmalemma and 25–30 Å for a vesicle approaching an identical vesicle. These results are used to explain why fused or aggregated vesicles are seen less frequently than isolated vesicles in the central cytoplasmic region of mammalian endothelial cells in electron micrographic studies.