The corrosion of steel and copper in contact with ester based fluid formulations in the presence of water at 150 and 60 degrees C has been investigated using impedance measurements, infra-red spectroscopy and weight change measurements. At 60 degrees C films are formed where the outer layers are loosely adherent and easily removed from the metal surface. At 150 degrees C, where the water content of the oil is much lower than at 60 degrees C, the films are much more adherent. The films consist largely of metal hydroxides at both temperatures. In the absence of oxygen very little film formation occurs.