Density functional calculations are performed on M 3 (OH) 7 (H 2 O) 6 2 + and M 3 O(OH) 6 (H 2 O) 6 + clusters for M Al, Cr(III), and Fe(III), allowing determination of the relative acidities of the μ 3 -hydroxo and aquo functional groups. Contrary to previous predictions and rationalizations, Fe 3 OH and Al 3 OH groups have nearly the same intrinsic acidity, while Cr 3 OH groups are significantly more acidic. The gas-phase acidity of the Fe 3 OH site is in good agreement with the value predicted by the molecular mechanics model previously used to estimate the relative acidities of surface sites on iron oxides. [J. R. Rustad et al. (1996) Geochim. Cosmochim. Acta 60, 1563]. Acidities of aquo functional groups were also computed for Al and Cr. The AlOH 2 site is more acidic than the Al 3 OH site, whereas the Cr 3 OH site is more acidic than the CrOH 2 site. These findings predict that the surface charging behavior of chromium oxides/oxyhydroxides should be distinguishable from their Fe, Al counterparts. The calculations also provide insight into why the lepidocrocite/boehmite polymorph is not observed for CrOOH.