The frictional behaviors of a variety of fatty alkenyl esters and their corresponding fatty epoxide esters (epoxy methyl oleate (EMO) and methyl oleate (MO), epoxy methyl linoleate (EMLO) and methyl linoleate (MLO), epoxy methyl linolenate (EMLEN) and methyl linolenate (MLEN)), epoxidized soybean oil (ESBO), and a commercial epoxidized 2-ethylhexyl transesterified soybean oil (VF) as additives in hexadecane have been examined in a boundary lubrication test regime using steel contacts. Langmuir critical additive concentrations were determined, which provide the following order of negative adsorption energies: ESBO > VF > EMO >= EMLO > EMLEN and MLEN >= MLO > MO. Thus, for the similar epoxidized materials the greater degree of epoxidation results in less negative calculated total adsorption energies; this trend is reversed for the olefinic parent systems. This ordering agrees with that obtained via a more complex unconstrained cooperative interaction adsorption model. Fits of the steady-state coefficient of friction (COF) versus concentration data indicate an inverse relation of the obtained interaction parameters (a) with the primary adsorption energies (E). These results demonstrate the complexity of the adsorption mechanism that occurs.