Fluorescence characteristics of 5-aminoindazole (5-INH2) have been studied at various acid-base concentrations in sodium dodecyl sulfate and cetyltrimethylammonium bromide (CTAB). The decrease in the fluorescence quantum yields of 5-INH2 in CTAB at pH 9 is partly due to the effect of the medium and partly due to bromide ion-induced fluorescence quenching. Interaction of this amine with detergent molecules at pH 9 leads to premicellar formation. Prototropic reactions in the micellar interface in S-0 and S-1 states involve the protonation and deprotonation of the amino group only. Biprotonic phototautomerism of singly protonated species is not observed in the micellar interface. Apparent pK(a) values, which were determined for both equilibrium reactions, are discussed.