A Re(I) carbonyl complex derived from 2-(2-pyridyl)-benzothiazole (pbt), [Re(H2O)(CO)(3)(pbt)](CF3SO3) (1), rapidly releases CO under low-power UV illumination. CO photorelease from 1 is accompanied by a change in luminescence from orange to deep blue. These two distinct luminescence signals have been successfully employed to track (a) the entry of the pro drug 1 into cancer cells and (b) the end of the CO (drug) delivery step within the target.