A [(bpy)Re(CO)(3)L](+) complex (bpy = 2,2'-bipyridine) in which L contains an azacrown ether (MacQueen, D. B.; Schanze, K. S. J. Am. Chem. Soc. 1991, 113, 6108) acts as a reversible light-controlled switch of alkali and alkaline earth metal cations bound to the azacrown, as observed directly by time-resolved UV-vis spectroscopy. Excitation to the metal-to-ligand charge-transfer (MLCT) state of the metal-complexed form, [(bpy)Re(CO)(3)L](+)-Mn+, results in cation release on the nanosecond time scale for Mn+ = Li+, Na+, Ca2+, and Ba2+. with Li+ and Na+ being released more rapidly than Ca2+ and Ba2+; by contrast, Mg2+ is not released. After decay to the ground state, [(bpy)Re(CO)(3)L](+) recaptures metal cations on the microsecond time scale to restore the starting, thermal equilibrium. A multistep rebinding mechanism is observed for Li+ and Na+, in which the cation attaches initially to the azacrown nitrogen atom before binding to the equilibrium position within the azacrown ring. The excited states and other intermediates in the cation release-and-recapture cycle have been observed directly in real time, and their decay rate constants have been determined as a function of cation identity, enabling a generalized light-controlled cation-switching mechanism to be developed for this generic molecular design.