Steam turbine, high pressure, electrohydraulic control systems have been successfully operated on chlorinated aromatic fire resistant fluids containing small amounts of aryl phosphate esters and on pure phosphate ester fluids. Because the chlorinated fluids caused servovalve sticking due to corrosion effects, phosphate esters were substituted, whereupon some cases of severe servovalve erosion resulted. The use of fuller's earth filtration solved the erosion problem completely, and there have been no further problems in the last four years where recommended fitration procedures were followed. Available data indicate that the erosion was due to residual, impurity level, chlorinated materials in the phosphate ester, operating through an erosion-corrosion mechanism. Principal evidence was the presence of chlorine in the eroded areas of the servovalve, as detected by electron microprobe analysis, the presence of chlorine in all erosive fluids and isolation of acidic and chlorine-containing compounds from fuller's earth cartridges that had stopped erosion. There was no evidence for other erosion mechanisms. The applicability of various chemical analysis techniques to hydraulic fluid problems is demonstrated.