The reactions of early lanthanide metal atoms Nd, Sm, and Eu with water molecules have been investigated using matrix isolation infrared spectroscopy and density functional calculations. The reaction intermediates and products were identified on the basis of isotopic labeled experiments and density functional frequency calculations. The ground state metal atoms react with water to form the M(H2O) and M(H2O)(2) complexes spontaneously on annealing (M = Nd, Sm, Eu). The M(H2O) complexes isomerize to the inserted HMOH molecules under red light irradiation, which further decompose to give the metal monoxides upon UV light irradiation. The Nd(H2O)(2) complex decomposes to form the trivalent HNd(OH)(2) molecule, while the Sm(H2O)(2) and Eu(H2O)(2) complexes rearrange to the divalent Sm(OH)(2) and Eu(OH)(2) molecules under red light irradiation.