The photocurrent responses of the oxide films formed on 304L stainless steel in high temperature and high pressure water was investigated in borate buffer solutions. The photocurrent responses showed transient characteristics. The oxide films exhibited a p-type semiconductor but presented negative steady photocurrents and positive photocurrent spikes under certain applied bias and incident light. A theoretical model was developed to analyze the transient photocurrent behaviors. This model was derived on the basis of continuity equation for carriers and can give a reasonable explanation for the transient photocurrent waveform patterns. The transient photocurrent waveform pattern is mainly dependent on the electric field inside the space charge region of the outer layer at oxide film/electrolyte interface. The effects of applied bias and incident light on photocurrent waveform patterns result from their impacts on the electric field. The values of calculated electric field by fitting the measured photocurrent data show a good agreement with the prediction of the model. (C) 2012 Elsevier Ltd. All rights reserved.