Association of water soluble anionic dyes and different types of surfactants has been studied in aqueous solutions. The anionic dyes were Remazol Red F3B and Remazol Brilliant Yellow GNL. Surfactants belonging to different classes were studied: anionic (SDS), cationic (DTAB), nonionic (OPE09), and zwitterionic (DDAPS). The two dyes were shown to aggregate in water when their concentration is above a threshold value. Evidence for the formation of mixed micelles could be obtained by means of H-1 NMR experiments. The surface tension lowering and critical micellar concentration (cmc) values of dye-surfactant mixtures were interpreted on the same grounds as those for surfactants mixtures. The regular solution theory originally proposed by Lange and Beck was used, the threshold concentration value for dye aggregation being considered as a cmc value. Deviations from ideal behavior were observed and evaluated, depending on the type of surfactant. Precipitation was observed with mixtures of anionic dyes and cationic surfactants, indicating strong interactions between them. Experiments indicated a strong synergism with a zwitterionic surfactant, and a moderate deviation from ideal behavior with anionic and nonionic ones. NMR experiments have shown that dyes are located at the interface in the mixed micelles. Some correlations between solubilization site and synergism in mixed micelle formation could be disclosed. (C) 1994 Academic Press, Inc.