Technical Abstract: The frictional behaviors of methyl oleate (MO), methyl palmitate (MP), methyl laurate (ML), and methyl stearate (MSt) as additives in hexadecane have been examined in a boundary lubrication test regime using steel contacts. It was found that the transient attributes of Coefficient of Friction (COF)-time spectra are a sensitive measure of adsorption equilibria. Critical additive concentrations were defined and used to perform novel and simple Langmuir analyses that provide an order of adsorption energies: MSt > MP > MO greater than or equal to ML. Comparison of Langmuir, Temkin, and Frumkin-Fowler-Guggenheim (FFG) adsorption models via non-linear fitting demonstrates the necessary inclusion of cooperative effects in the applied model. In agreement with the qualitative features of steady-state COF-concentration plots, MSt displays minimal cooperative adsorption. MO, MP, and ML data require large attractive interaction terms (beta greater than or equal to 4) to be adequately fit. These results, and comparison with previous adsorption studies of MO and MSt suggest that reordering of adsorbed molecules, at increased additive concentrations, rather than specific intermolecular interactions determines the observed cooperative behavior. Primary adsorption energies calculated via the cooperative model are necessarily decreased while total adsorption energies correlate well with values obtained via critical concentration analyses.