Optical Interferometry is used to study the film thickness of greases, and of soap suspensions and of their base oils in point or line contact. In one case, samples were taken from a grease-lubricated plain-bearing test after 5 1/2- and 25-million revolutions. The effective viscosity of a grease under the very high shear rates of the order of 1/2 × 10-6s-1encountered in rolling contacts can be calculated.Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE Lubrication Conference in San Francisco, California, August 18–21, 1980.