The effect of additives-base oil molecules interactions, in combination with surface properties and operational conditions, on the tribological behavior of base oil-additive blends was studied. Different model base oils (n-hexadecane and diethylene glycol dibutyl ether) and additives (C4-zinc dialkyl dithiophosphates and oleic acid), were used. The diffusion coefficients of ZnDDP were dependent on temperature and pressure, however, the two base oils showed comparable values. The different tribological performance of additive/base oil systems was due to the competition between molecules for a place on the steel surface. The base oil-additives interaction influenced the transport of additives to the rubbing surfaces and thereby the growth rate and kinetics of formation of the additive-derived reaction layers. The interaction also affected the morphology and mechanical properties of the reaction films, leading to the differences in friction and wear behavior. The intensity of the tribological contacts accelerated the formation of tribological reaction layers. The applied shear stress in tribological contacts influenced the way additives adsorb onto the rubbed surfaces. This is an abstract of a paper presented at the World Tribology Congress (Kyoto, Japan 9/6-11/2009).