The oxygen evolution reaction was studied by electrochemical impedance spectroscopy in basic medium. The experimental spectra show two faradaic time constants. The appearance of these two time constants led us to propose a reaction mechanism with two intermediate iridium oxides. A theoretical model to account for the electrochemical impedance of the proposed model was developed. This model reproduces the features of the experimental spectra. Therefore, it can be concluded that, probably, the oxygen evolution reaction follows the proposed mechanism.