A benzothiazolium styryl azacrown ether dye (1) and its complexation with Ba2+ and Na+ cations in acetonitrile solution have been studied by UV-vis absorption and emission spectroscopy, trans-1 is found to act as a normal chromoionophore for Ba2+ and Na+ at high concentration (greater than or equal to 10(-2) M) by complexation with the azacrown and as a light-controlled, transient chromoionophore specific for Ba2+ at low concentration (10(-5)-10(-3) M). A quantitative analysis of the complexation and thermal reactions following photolysis has enabled a detailed mechanism to be proposed : trans-1 photoisomerization produces cis-1, which is stabilized in the presence of Ba2+ by dual intramolecular complexation of the cation with both azacrown and sulfonate groups.