By using a combination of spacer layer optical interferometry and spectral analysis, it has recently become possible to measure lubricant films down to less than 2 nm thickness in high-pressure, rolling ball on flat contacts. This enables the presence and some of the properties of boundary lubricating films to be observed directly for the first time in realistic contact conditions. This paper describes some recent work using this approach. It is shown that highly refined, nonpolar base fluids exhibit classical hydrodynamic behavior down to less than 2 nm film thickness, whereas fluids such as esters give enhanced film thickness in this very thin film region. Work on polymer solutions and base fluid mixtures suggests that the main way that such fluids form boundary layers is by preferential adsorption on solid surfaces of polar moieties from solution. This has the effect of changing the composition and thus effective viscosity of lubricant layers close to solid surfaces, thereby leading to a hydrodynamic response different from that predicted from the bulk rheological properties of the blend.