NMR self-diffusion coefficient measurements have been used to study the properties of polyethylene glycol (23) lauryl ether (Brij-35) with cetyltrimethylammonium bromide (CTAB) in the mixed aqueous solutions with different mole fractions of CTAB. By fitting the self-diffusion coefficients to the two-state exchange model, the critical micelle concentrations of the two solutes in the mixed solutions (cmc(1)* and cmc(2)*) were obtained. The critical mixed micelle concentrations (cmc*) were then evaluated by the sum of cmc(1)* and cmc(2)*, which are in good agreement with the results measured by the Surface tension method. The cmc* values are lower than those of the ideal case of mixing. which indicates that the behavior of the CTAB/Brij-35 system is nonideal. Moderate interactions between CTAB and Brij-35 in their mixtures can be deduced from the interaction parameters (beta(M)) based on the cmc* obtained by the NMR self-diffusion method. The compositions (x(1)) of the mixed micelles at different total surfactant concentrations were also evaluated. By using these results, a possible mechanism of mixed micellar formation and a picture of the formation of nonsimultaneous CTAB/Brij-35 binary mixed micelle were proposed. In contrast to the case of CTAB/TX-100 system, Brij-35 molecules have a tendency to form micelles first at any mole fraction of CTAB. The mixed micellar self-diffusion coefficients (D-m) increase slightly at lower CTAB molar ratios, and then speed up with increasing CTAB mole fraction. (C) 2004 Elsevier Inc. All rights reserved.