Data are presented on the changes in the properties of a disiloxane/ester fluid resulting from water concentrations as small as 200 ppm while undergoing mechanical stress at 275 F in a simulated aircraft hydraulic system. The response of fluid flash point, viscosity, neutralization number, and hydraulic system filter differential pressure are discussed for hydraulic fluid water concentrations as high as 18,900 ppm. The efforts of previous investigators, employing bench tests and simulated systems, are reviewed. The limitations and sources of error of methods of determining fluid degradation and level of water contamination, as determined by this study, are noted. Attention is given to the importance of reacted water, for which no routine method of determination has been developed, as apposed to free water as determined by the Karl Fischer method of analysis.Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE International Lubrication Conference held in New York City, October 9–12, 1972