Abstract: In acidic medium the intcrreaction of basic dye cations and (or) ion-assoication complexes with r.onionic surfactants (such as Emulsifying agent OP) has been studied. On the basis of comparis on and analysis of corresponding spectra the effect of nonionic surfactants on the micellar solubilization spectrophotometry is described as "PseudohomogeneousExtraction Model" ——the ion-association complexes do not react withthe individual molecules of nonionic surfactants or the micelle, they are only solubilized in the polyethoxy group of nonionic surfactants micelle. This conclusion has been further confirmed by phase separation experiments at the cloud point and by organic solvent extraction.The effect of hydrogen ion concentration [H⁺] on the decolourizing rate and protonation of basic dye, on the equilibrium and sensitivity of the ion-association complexes has also been discussed. In the presence of nonionic surfactants the reagent blank shows a light-green colour which has λ_max =620nm,but the absorbance maxima of the ion-association complexes (CV-PMo and CV-AsMo) which show a dark-blue colour, are all located at 545nm.The method has been used for the spectrophotometric determination of Pand As in the steel.