The dissolution behavior of anodic oxide films formed on aluminum in phosphoric acid solutions has been studied by impedance and potential measurements. The dissolution follows a first order mechanism. Results indicate that the film is of duplex nature. Changes in current density (CD) during anodizing apparently affect the thickness of the outer layer only. The dissolution of the outer layer increases with increasing acid concentration, while the inner layer is not affected in the same degree by changes in acid concentration. The influence of some anions on the rate of dissolution of the oxide indicates that among PO43−, SO42−, NO3−, l−, Br−, and ClO4−, the last two are the most aggressive. The rate of dissolution in the presence of different anions is related to the resulting field strength it induces by each across the oxide as well as to its adsorption properties.