Successive water addition to the mononuclear species SO3(g) and ClO3(OH)(g) and ArO4(g), as well as the binuclear clusters S2O6(g) and Cl2O7(g), is Studied by means of quantum chemistry. A large basis set effect is observed for the S-O, Cl-O, and Ar-O bond distances, as well as for redox energetics. While the 6-311+G(d,p) basis set is enough for O and H atoms, S, Cl, and Ar require the 6-311 +G(3df) set for an accurate description. Given this combination of basis sets, B3LYP is found to give very good results of equal quality to high-level methods. The dominating chemical feature that is monitored is the instability of the S=O, Cl=O, and Ar=O bonds. This instability causes the acidity displayed by water solutions of SO2(OH)(2) and ClO3(OH) and makes the Cl and Ar systems prone to O-2(g) evolution. The latter system is also reduced directly by water, while the reduction of SO2(OH)(2) to H2S requires a reducing agent of equivalent power to H,. Three water ligands are necessary to produce nanoscale deprotonation f SO2(OH)(2) and ClO3(OH). The hydrolytic cleavage of the S-O-S and Cl-O-Cl bridges is exothermic by 23-30 kJ/mol, which explains the sensitivity to excess of water displayed by these polymers.