The lateral diffusion of acridine orange was investigated at the interface of water and octadecylsiloxane (C-18) monolayers of varying hydrocarbon density; The surfaces examined ranged from 24% to 100% of a sterically complete monolayer of 3.5 mu mol/m(2) in alkane coverage. Fluorescence recovery after photobleaching was used to measure the lateral diffusion coefficient. Acridine orange is a fluorophor known to reside at the water/hydrocarbon interface, negligibly adsorbing to bare silica. Its diffusion coefficient was found to decrease 500-fold as the surface coverage was reduced 4-fold. By use of effective medium and percolation theory, the strong density dependence was attributed to decreased contiguity of hydrocarbon at lower coverages. The temperature dependence of the lateral diffusion coefficient showed that the decease in diffusion rate at lower coverage was entirely due to an increase in the entropic barrier, pointing to percolation as the mechanism of diffusion.