Molecular dynamics simulations have been used to examine the friction between the hydrogen-terminated (111) faces of diamond with small hydrocarbon (third-body) molecules trapped between them. In general, the presence of the trapped third-body molecules reduced the friction between the diamond surfaces with the most pronounced reduction at high loads. The size and shape of the third-body molecule, as well as the alignment of atoms on opposing diamond surfaces, were found to be paramount in determining the magnitude of the friction. These results are compared to results from previous simulations that examined the effects of chemically bound hydrocarbons on the friction between diamond surfaces and to available experimental data.