Light-induced metastable linkage isomers of trans-[Ru(NH3)(4)Cl(SO2)]Cl and trans-[Ru(NH3)(4)(H2O)(SO2)](C6H5SO3)(2) have been identified for the first time using photocrystallographic methods. In both linkage isomers the SO2 ligand is side bound, but the Ru-O and Ru-S distances are considerably longer and almost equal in the trans-H2O isomer. DFT calculations confirm that both isomers correspond to minima on the ground-state potential energy surface and also predict the existence of a second oxygen-bound isomer for both compounds. The decay of the light-induced species has been studied by both DSC and I R. Activation energies for the thermal back-reaction, as derived from the temperature-dependent disappearance of light-induced IR bands, are 50.0 and 58.4 kJ/mol for the two isomers, which is larger than the corresponding numbers for photoinduced side-bound nitrosyl linkage isomers.