Ga-2 reacts spontaneously with H-2 in solid Ar matrixes at 12 K to form the cyclic molecule Ga(mu-H)(2)Ga. In-2 does not react with H-2 under similar conditions, but irradiation at wavelengths near 365 nm induces the formation of the corresponding indium hydride, In (mu-H)(2)In. The molecules have been identified and characterized by the IR spectra displayed by matrixes containing the metal and H-2, D-2, HD, or H-2 + D-2; they each have planar, dihydrido-bridged structures with D-2h symmetry, as endorsed by comparison of the measured spectra (i) with the properties forecast by quantum chemical calculations and (ii) with the spectra of known gallium and indium hydrides. Both are photolabile under visible light (lambda > 450 nm): green light (lambda = ca. 546 nm) causes Ga(mu-H)(2)Ga to isomerize to a mixture of HGaGaH and H2GaGa, whereas broad-band visible irradiation (lambda > 450 nm) of In(mu-H)(2)In gives rise to the isomer HInInH, together with InH. The isomerization can be reversed by UV photolysis (lambda = ca. 365 nm) of HGaGaH, H2GaGa, and HInInH or by near-IR photolysis (lambda > 700 nm) of HGaGaH and H2GaGa.