Ultraviolet‐visible (UV‐vis) absorption, flash photolysis, and 1H and 13C NMR spectroscopy were used to investigate the mechanism of formation and structure of complexes in the 1',3',3'‐trimethylspiro2H‐1‐benzopyran‐2,2'‐indoline (SP, also well known as BIPS) with Al3+ (inorganic salts) in ethanol (EtOH), methanol (MeOH), and in aqueous MeOH solutions. A labile (equilibrium constants ≤ 5000 M−1) complex of SP and Al3+ with broad absorption band in the UV‐vis with λmax = 380 nm appeared promptly in the presence of an excess of Al3+. The slow formation of a stable complex (SC) between Al3+ and two merocyanine (MC) forms of SP with an intensive absorption band at λmax = 430 nm is observed with a yield of 1.0 upon keeping the solutions of these two compounds at constant concentration ratio Al3+ ≥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 Al3+ at concentrations of the same order as those of MC. This method allows detection of Al3+ to concentrations as low as 50 nM. The kinetics and thermodynamic parameters of the SP and MC reactions and of their complex formation with Al3+ are discussed. A new stable colored complex of a merocyanine form of 1',3',3'‐trimethylspiro2H‐1‐benzopyran‐2,2'‐indoline with Al3+ was observed. Spectroscopic, kinetic, and thermodynamic properties of the study systems are presented. The developed method allows detection of Al3+ at concentrations as low as 50nM.