The load-carrying properties of a range of dialkyl phosphites have been examined under antiwear (AW) and antiscuff conditions using the four-ball machine. Adsorption/reaction of these compounds with iron surfaces has also been carried out. The results showed that both the load-carrying and adsorption/reaction properties of these dialkyl phosphites are markedly affected by the chain length and structure of the alkyl group. A mechanism of action of dialkyl phosphites is proposed which not only provides an explanation for both sets of observations but also highlights the difference in load-carrying action of dialkyl phosphites compared to the corresponding phosphates.Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference held in Atlanta, Georgia, October 16–18, 1973