Mixed micelle formation and synergistic interactions of binary surfactant combinations of sodium nonylphenol polyoxyethylene ether sulfate (NPES) with typical surfactants such as sodium dodecyl sulfate (SDS), Triton X-100 (TX100), cetyl trimethyl ammonium bromide (CTAB), and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) at 25 degrees C in the presence of NaCl have been investigated. The critical micelle concentration of the binary mixtures has been quantitatively estimated by steady-state fluorescence measurements. The micellar characteristics such as composition, activity coefficients, and mutual interaction parameters have been estimated following different theoretical treatments. Investigation on the micellization and synergistic interaction of NPES with four kinds of surfactants showed that the behavior of the binary mixture deviated from the ideal state. The analysis revealed that the interaction parameter values (beta) varied with variation of solvent composition. Besides the strong electrostatic attraction between the oppositely charged surfactant NPES-CTAB mixture, the interaction between NPES and SDS also showed far more deviation from ideal behavior than that of TX100 and AOT. The reason for the synergism is also discussed and the results show that an ionic and a nonionic surfactant character existed concurrently in NPES due to the combination of a sulfate group and polyoxyethylene as a hydrophilic moiety. Zeta potential and diffusion coefficient measurements of micelles confirmed the synergistic interaction between the binary surfactants.