Gas-phase acidities and aqueous solution pK(a)'(s) are predicted for MO,(OH)2, where the center, atom M is a main Group 6, 16, and U atom using the Feller Peterson Dixon approach based on coupled cluster CCSD(T) calculations with additional corrections. The gas-phase acidities of the MO,(OH)(2) compounds are essentially the same for elements (M) of the same group, 304-310 kcal/mol at 298 K. All of the Group 6 compounds are 5-6 kcal/mol less acidic in the gas phase than H2SO4. The gas phase acidity of UO2(OH)2 is calculated to be up to 338.0 kcal/mol,,similar to 10% less acidic in the gas phase than the other MO2(OH)2 acids. The most acidic molecule in aqueous solution is predicted to be H2SO4. Overall, for the Group 16 compounds, the pKa's increase going down the group, with H2PoO4 predicted to be slightly more acidic than nitric acid. H2CrO4 is the most acidic of the Group 6 transition metal compounds. The aqueous acidities of H2MoO4 and H2WO4 are comparable and about 3 pK(a) units less acidic than H2CrO4 and comparable in acidity to HNO3. H2UO4 is not acidic at all in aqueous solution with a plc near 20 plc units and is also not predicted to readily undergo hydrolysis reactions.