An X-ray technique is described for the measurement of the thickness and shape of thin oil films (from 5 to 50 millionths of an inch thick) formed between the rolling and rolling-sliding surfaces of hardened steel rollers as in rolling bearings and gears. The film thickness with white mineral oil, diester-base, and silicone lubricants was found to vary with temperature (viscosity), speed, and load in much the same way as expected from elasto-hydrodynamic considerations. The shape of the elastically flattened contact regions on the rollers appeared to change with rolling speed in such a way as to explain why ball bearings and gears have longer fatigue life (in revolutions) at high speed than at low speed.