The influence of the type and the concentration of several mineral acids (HNO3, H2SO4, and HCI) on the dissolution of two different chromium (hydrous) oxides was investigated at 25 and 65 degrees C. The results were interpreted using a combined approach that takes into account the electrochemical nature of the process, the surface complexation, and multiple-site models for the dissolution of very passive covalent oxides. The attack by protons on oxo bonds is the critical step that strongly enhances the rate of the process at higher acidities. The reactivity of chromium (hydrous) oxides is shown to be related to the degree of cross-linking and not to the water contents.