Experiments are described in which elastohydrodynamic and boundary lubrication properties of O/W emulsions of mineral oil are studied with a variety of nonionic surfactants as the emulsifying agents. The changes in the minimum elastohydrodynamic film thickness are determined with different surfactants, with changes in oil concentration, with changes in emulsifying agent concentration, and with blending of different surfactants. These results can be quantitatively explained by a theory which assumes trapping of oil particles between steel surfaces with a certain probability. It is then shown that this trapping probability is correlated to the 'displacement energy' at the oil-water-steel boundaries. On the other hand, boundary lubrication properties of the emulsions are exclusively governed by those of the surfactants themselves suggesting that, under these lubricating conditions, their adsorbed films play a dominant role.Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Baltimore, Maryland, October 16–19, 1988