The photochemical behavior of the trans- and cis-[] complexes has been investigated in organic coordinating solvents (dmso, ) and aqueous solutions by means of electronic and NMR spectroscopy as well as chloride-selective electrode measurements. Excitation in the UVA and visible region of the cis-[] complex in dmso leads to geometric isomerization with quantum yields and to give the photostable trans complex, whereas in acetonitrile and aqueous solutions, both isomerization and substitution processes occur. Moreover, in the latter two solvents, the trans isomer is photoactive and undergoes substitution reactions. In acetonitrile, for both trans- and cis-[] isomers, selective photolabilization of the dimethylsulfoxide ligands results in the formation of the trans-[] complex. In aqueous solutions, the dmso and ligands are gradually substituted by water molecules to give as a final product a mixture of(aqua)ruthenium(II) and (aqua)(chlorido)ruthenium(II) complexes. These species may prove to be useful in the binding of cellular components.