The action of lubricant additives has been analysed under boundary lubrication on metal couples of Cr-bearing steel plates and cast iron pins in reciprocating mode using model oil systems. Tribological parameters have been measured in addition to analyses of the wear films lubricated with different oils: base oil, oil+ZDDP (Zinc Dialkyldithiophosphate), oil+ZDDP plus addition of Ca-sulphonate detergent and oil+MoDTP (Molybdenum Dithiophosphate). Isothermal, constant load tests have been carried out using a pin-on-reciprocating plate apparatus under lubricated wear conditions at a bulk oil temperature of 100°C. Tests where load has been progressively increased have also been conducted. The friction and wear analysis of one and eight hours tests were carried out. The chemical nature of the wear film formed by different oils at varying times and with varying loads has been examined by Energy Dispersive X-Ray analysis (EDAX) which penetrates a few µm into the sample. X-ray Photoelectron Spectroscopy (XPS) was used to analyse the top few nm of the surface, focussing mainly on the wear film chemistry. The chemical composition of the different films were analysed at different depths as the specimen was progressively etched with an argon ion beam at 2kV. It has been shown that the nature of the wear film is directly affected by the nature of the lubricant additives and these additives are affected to a different degree by the load. A direct correlation between the nature of the boundary film and tribological response can be seen and both show the strong interaction between the additives.