Previous studies have shown that photoelectrochemical methods used to study the semiconducting properties of oxides may be extended to in situ investigations of thin oxide films. A new expression relating the photocurrent to the film thickness has been derived by combining a simplified Gartner/Butler model and an optical model for multiple internal reflections. With this model it is shown that the observed dependence of the photocurrent on film thickness is caused by multiple internal reflections. There is a good fit to experimental data for TiO2 films illuminated at 350 nm and under moderate bias. The relevance of multiple internal reflections to the analysis of photoelectrochemical microscopy data is also discussed.