[Ru(bpy)2dmdppz](2+) (bpy = 2,2'-bipyridine and dmdppz = 3,6-dimethyl dipyridylphenazine), a strained Ru(II) polypyridyl complex, is a derivative of the well-known luminescent "light switch", [Ru(bpy)2dppz](2+) (dppz = dipyridylphenazine). [Ru(bpy)2dmdppz](2+) is of interest because it acts as a photochemical sensor and metalating agent for DNA. Here we report a detailed study to elucidate the mechanism of ligand substitution by investigating the photochemical reaction in a variety of solvents and by determining the effects of different incoming ligands, the incoming ligand concentration, and the temperature dependence. Results from these studies indicate that the mechanism of substitution is associative or interchange associative, in contrast with the dissociative mechanism of other photolabile Ru(II) polypyridyl complexes.