The frictional behaviour between mild steel surfaces lubricated with excess of pure hydrocarbons, ketones, alcohols, amides, acids and esters has been investigated at low speeds and under high loads. In all cases a transition from smooth sliding to irregular stick and slip motion takes place at a temperature characteristic of the lubricant employed. Experiments in which lubricant films one or more molecules thick were built up by the Langmuir-Blodgett technique have shown that the transition from smooth sliding to stick-slips occurs when the adsorbed surface film of lubricant breaks down and becomes disoriented. Acids and esters are shown to be strongly adsorbed, while hydrocarbons, ketones, alcohols and amides are not appreciably adsorbed. It is shown that adsorption of acids and esters occurs by the interaction of the dipoles in their polar group with the metal atoms in the surface. The results also suggest that molecules of long-chain compounds are oriented on a metal surface in the same way as they have been shown to be arranged on an aqueous surface. Measurement of the coefficient of friction between surfaces lubricated with films one and many molecules thick has shown that under conditions of 'boundary lubrication' prevailing at high loads and low speeds, excess of lubricant is squeezed out, and lubrication is effected by a unimolecular film adsorbed on each surface. The variation of the coefficient of friction with load in the case of oleic acid shows that orientation with this lubricant extends beyond the primary adsorbed layer. This result accounts for the low values of the coefficient of friction obtained by previous workers, and explains the good lubricating properties of oleic acid. These experiments show that a study of the frictional behaviour provides a method of investigating the properties of surface films on metals.