Infrared absorptions for the matrix-isolated lead and tin hydroxides M(OH), M(OH)(2) and M(OH)(4) (M = Pb, Sn) were observed in laser-ablated metal atom reactions with H2O2 during condensation in excess argon. The major M(OH)(2) product was also observed with H-2 and O-2 mixtures, which allowed the substitution of O-18(2). The band assignments were confirmed by appropriate D2O2, D-2, (OO)-O-16-O-18, and O-18(2) isotopic shifts. MP2 and B3LYP calculations were performed to obtain molecular structures and to reproduce the infrared spectra. The minimum energy structure found for M(OH)(2) has C, symmetry and a weak intramolecular hydrogen bond. In experiments with Sn, HD, and O-2, the internal D bond is favored over the H bond for Sn(OH)(OD). The Pb(OH)(4) and Sn(OH)(4) molecules are calculated to have S-4 symmetry and substantial covalent character.