Commercial airlines use a variety of hydraulic fluid mixtures, some of which had been reported to cause increased valve erosion. An extensive test program was conducted to determine why certain mixtures of aircraft phosphate ester hydraulic fluids cause erosiveness in valves while others have negligible effect. Use of nonerosive mixtures is important to the aircraft industry to reduce the high cost of maintenance caused by erosion. Accelerated tests were conducted to identify erosive and nonerosive mixtures of presently available fluids. Chemical tests were conducted with a matrix of test fluids to establish methods that will predict the effects measured in erosion tests. Erosiveness of fluid mixtures can now be predicted by means of two laboratory measured electrochemical properties; wall current and threshold corrosion current density. This paper discusses the theory of electrochemical erosion, describes the tests conducted to classify erosion characteristics of fluid mixtures and describes the laboratory test methods that can be used to identify the erosion potential of mixtures.