The microsurface adsorption - spectral correction (MSASC) technique has been applied to the interaction of indigo carmine (IC) with cetyltrimethylammonium bromide (CTAB). The aggregation of IC on CTAB obeys Langmuir isothermal adsorption. The results show that both the monomer complex $IC{cdot}CTAB$ and the micellar complex $(IC{cdot}CTAB)_{78}$ were formed. The binding constant of the monomer complex was calculated to be $K_{IC{cdot}CTAB}$ = 2.20 ${ imes}10^5L{cdot}mol^{-1}$, and the molar absorptivity of the micellar complex was calculated to be ${varepsilon}_{(IC{cdot}CTAB)78};^{560nm}$ = 8.58 ${ imes}10^5L{cdot}mol^{-1}{cdot}cm^{-1}$. The aggregation was applied to the determination of cationic surfactant in wastewater.