A benzothiazolium styryl azacrown ether dye (1) and its photocontrolled complexation with Ba2+ in acetonitrile solution have been studied by nanosecond time-resolved UV-visible absorption spectroscopy. Continuous visible irradiation of trans-1 in the presence of Ba2+ was used to generate a photostationary-state mixture with ''closed'' cis-1-Ba2+ present in high concentration: in this form, the barium cation is complexed with both the azacrown and a propylsulfonate group pendant to the benzothiazolium dye. A nanosecond UV photolysis pulse was used to induce photoisomerization of this ''closed'' cis-1-Ba2+ species. Time-resolved UV-visible absorption studies have revealed that the barium cation initially remains complexed to the azacrown in the trans-isomer formed <50 ns after UV photolysis and that the cation is subsequently released from the azacrown on a time scale of ca. 700 ns, consistent with the equilibrium composition of the solution expected from the thermal chemistry under these conditions. Along with time-resolved and steady-state studies of the effect of temperature and cation concentration, these studies have enabled a quantitative mechanism to be proposed for the thermal and photochemical ion-complexing properties of this dye. These direct observations demonstrate that sustained, photocontrolled ion release from an azacrown ether dye can be achieved within 1 mu s of photolysis.