IR and UVâ vis spectra have been used to chart the thermal and photolytic reactions occurring during or after the codeposition of Ga vapour with an excess of H2O-doped Ar. Following the formation of the loosely bound adducts Gaâ ¯OH2 and Ga2â ¯OH2, detected after deposition of the matrix, UV irradiation (200 â ¤ λ â ¤ 400 nm) causes Gaâ ¯OH2 to rearrange with insertion of the metal atom into an Oâ H bond forming the Ga(ii) hydride HGaOH, while visible irradiation (λ = ca. 580 nm) causes Ga2â ¯OH2 to rearrange with the formation of cyclic Ga(μ-H)(μ-OH)Ga. Broad-band photolysis (200 â ¤ λ â ¤ 800 nm) results in the destruction of both these products, with HGaOH decaying to GaOH. Characterisation of the various molecules by IR spectroscopy has been underpinned by reference (i) to the effects of isotopic change (^1,2H, ^16,18O and ^69,71Ga) and (ii) to the results of density functional theory (DFT) calculations. The properties of the molecules are compared with those of related species, and the mechanisms of the photochemical changes are discussed.