A variety of organophosphorus and other selected compounds were studied as antiwear additives for neopentyl polyol ester lubricants on 440C stainless steel rubbing surfaces using a four-ball wear test machine. Their effectiveness as additives was compared with that of tricresyl phosphate. Although 440C steel is relatively unreactive, it was possible to improve strikingly its wear and friction characteristics with organophosphorus additives. Some of the best results were obtained with acid phosphates. Alkyl chain length also appeared to be a significant factor. It was demonstrated that small concentrations of extraneous acidic substances are responsible for the antiwear action of phosphites at high load and that larger amounts of such acidic materials are needed as the load increases. Experiments involving addition of amines to neutralize formulations containing an acid phosphate confirmed that free acids promote antiwear action at the higher loads. Vinyl stearate and selected compounds containing chlorine or sulfur proved ineffective as additives for 440C steel in the antiwear region.Presented at the 26th ASLE Annual Meeting in Boston, Massachusetts, May 3–6, 1971