The linear polarization or polarization resistance (PR) technique was investigated in high and low salinity geothermal brine at 150 and 250°C on a lowcarbon steel (A53B) and a ferritic stainless steel (E-Brite 26-1) with and without the presence of oxygen in the brine. There was good agreement between the weight loss data and the linear polarization data. Oxygen generally accelerates the corrosion rate, and in the presence of oxygen the usual form of the polarization resistance equation must be replaced by the mass transfer equation to predict the correct corrosion rates. The results were compared with the corrosion rate determined by commercial PR instrumentation, and it was concluded that this instrumentation is adequate for field investigations in which oxygen is absent. However, when oxygen is present, the commercial instruments may underestimate the actual corrosion rate by a considerable amount.