Abstract Ultraviolet‐visible (UV‐vis) absorption, flash photolysis, and 1 H and 13 C NMR spectroscopy were used to investigate the mechanism of formation and structure of complexes in the 1',3',3'‐trimethylspiro2 H ‐1‐benzopyran‐2,2'‐indoline (SP, also well known as BIPS) with Al 3+ (inorganic salts) in ethanol (EtOH), methanol (MeOH), and in aqueous MeOH solutions. A labile (equilibrium constants ≤ 5000 M −1 ) complex of SP and Al 3+ with broad absorption band in the UV‐vis with λ max = 380 nm appeared promptly in the presence of an excess of Al 3+ . The slow formation of a stable complex (SC) between Al 3+ and two merocyanine (MC) forms of SP with an intensive absorption band at λ max = 430 nm is observed with a yield of 1.0 upon keeping the solutions of these two compounds at constant concentration ratio Al 3+ ≥SP/2 in the dark. The rate constants of such SC formation were close to the corresponding rate constant of the transformation of SP into MC in the dark (5.0×10 −5 ‐1×10 ‐3 s ‐1 , depending upon the solvent). The photolysis of the SC with visible light (λ > 400 nm) results in the total conversion of the SC into SP. The SC forms promptly after the addition of Al 3+ at concentrations of the same order as those of MC. This method allows detection of Al 3+ to concentrations as low as 50 nM. The kinetics and thermodynamic parameters of the SP and MC reactions and of their complex formation with Al 3+ are discussed.