This study demonstrates that insight into the tribological reactions taking place in mixed and boundary lubrication can be provided by combining microsample four-ball wear tests with chemical analysis using gel permeation chromatography (GPC) as the principal analytical tool. At the end of the microsample four-ball wear test the lubricant turns into a grease-like mixture preventing the liquid lubricant from recirculating into the wear track and thus causing failure. Analyses of the various lubricant samples after their failure in the microsample test all show a relatively small amount of insoluble deposits and a large quantity of unreacted fresh lubricant. Virtually no intermediate reaction products were found. Combining this information with lubricant stability and the fact that a large quantity of lubricant flowed through the sliding junction while only a small portion was oxidized suggests that two very different thermal environments existin the concentrated contact. The insoluble deposits are typical of thermal oxidative reactions that require temperatures of400°C or above. The unreacted lubricant found at failure indicates that this portion of the lubricant sample was maintained at temperatures of 150°C or below. The formation of grease-like mixture with as little as four percent reacted material indicates the remaining liquid lubricant and its insoluble reaction deposits were well mixed throughout the test. These findings suggest that the hot zones causing severe lubricant degradation are in the immediate vicinity of the asperity-asperity contacts while the low temperature zone - the valleys between asperities which are in the majority - are much cooler.Presented at the 50th Annual Meeting in Chicago, Illinois May 14–19, 1995