The reactions of laser-ablated lanthanide metal atoms withperoxide or hydrogen plus oxygen mixtures have been studi experimentally in a solid argon matrix and theoretically with the ab initio MP2 and CCSD(T) methods. The Ln(OH)(3) and Ln(OH)(2) molecules and Ln(OH)(2)(+) cations are the major products, and the reactions to form those hydroxides are predicted to be highly exothermic at the CCSD(T) level. Vibronic interactions are hypothesized to contribute to the abnormalities in deuterium shifts for Ln OH(D) stretching modes for several hydro- xides, consistent with CASSCF calculations. Additional new absorptions were assigned as HLnO or LnOH and OLnOH molecules. The tetrahydroxides of Ce, Pr, and Tb have also been observed. These reactive intermediates were identified from their matrix infrared spectra by using D202, HD, D2, O-16,18(2),and O-18(2) Iotopic substitution, by matching observed frequencies with values calculated by electronic structure methods, and by following the trends observed in frequencies going through different lanthanide metal hydroxide series across the periodic table. The lanthanides are in the +II oxidation state for Ln(OH)(2) and are in the +III oxidation state for Ln(OH)(3) and Ln(OH)(2)(+).