The lateral diffusion coefficient of acridine orange was measured for a series of liquid alkane/water interfaces. The data show that the lateral diffusion coefficient differs among these interfaces and that lateral diffusion is slowed by interfacial roughness. Hydrocarbon viscosity had previously been suggested to play a role : analogous measurements for a C-18/water interface indicated that the reconfiguration of the hydrocarbon controls the lateral diffusion coefficient. However, the results of this work show that hydrocarbon viscosity does not significantly influence the lateral diffusion of acridine orange at liquid hydrocarbon/water interfaces. A simple mathematical model is developed to provide a plausible explanation for the observations that interfacial roughness slows lateral diffusion for liquid/liquid interfaces and hydrocarbon viscosity has less of an effect for liquids than for viscous monolayers.