Fluorescence recovery after photobleaching was used to characterize the diffusion of fluorescently labeled phospholipids at the oil/water interface for oil viscosities that varied over four orders of magnitude. Measurements were performed over a range of surface concentrations corresponding to molecular areas of 40-130 A(2)/molecule. As expected, the interfacial diffusion coefficient increased with molecular area, saturating at an area of approximately 100 A(2)/molecule. At molecular areas below approximately 80 A(2)/molecule, macroscopic domains of a condensed monolayer phase were observed; the diffusion of these domains was characterized by direct tracking and trajectory analysis. For oils with viscosity </=1500 cP, the diffusion coefficients of both individual probe molecules and condensed domains were consistent with a mechanism where the objects moved within the interface, experiencing drag from the adjacent bulk phases. Because this drag was dominated by the oil viscosity, the diffusion coefficients decreased proportionally to the inverse of the oil viscosity. However, for oils with higher viscosity, the diffusion coefficient of individual probe molecules decreased much more slowly. These observations suggested that two diffusive mechanisms are involved: one where surfactant molecules move within the interface and one that is analogous to the activated "hopping" processes that occur at the solid/liquid interface. This latter mode becomes significant only for very viscous oil phases.