Effects of micelle-to-vesicle transitions on the degree of counterion binding (beta) were investigated on three systems. For the concentration-dependent micelle-to-vesicle transition in the didodecyldimethylammonium bromide (DDAB)/water system, in the region of coexistent micelles and vesicles, less than 3 mM, the beta values increased significantly with DDAB concentration: beta(0.07 mM) = 0.35 and beta (3 mM) = 0.93. In the coexistent region, activities of the bromide ion, a(Br), were almost independent of the DDAB concentration, suggesting the pseudo-phase nature of both micelles and vesicles. In the concentration-dependent vesicle-to-lamellar transition region above 5 mM, where multilamellar vesicles were prevailing, on the other hand, the values were only little affected by this transition. This suggests that the increase in the layer number of DDAB multilamellar vesicles scarcely affects the values. This Was also Supported by the fact that the destruction of multilamellar vesicles by ultrasonication did not change the P values. These results strongly suggest that the inner and Outer monolayers of DDAB multilamellar vesicles are characterized by similar 6 values. The second system, cetyltrimethylammonium bromide (CTAB)/DDAB mixtures. showed composition-dependent transitions depending on the mole fraction of DDAB X-DDAB: spherical micelles (0 < X-DDAB < 0.2) -> rodlike micelles (0.2 < X-DDAB < 0.4) -> vesicles (0.6 < X-DDAB < 1). Values of beta increased with both the sphere-to-rodlike micelle transition and the micelle-to-vesicle transition. The increase in the value was larger for the latter than for the former transition. Little effects Of Ultrasoflication on was also observed for the multilamellar vesicles of CTAB/DDAB mixtures. In the third system of the mixtures of two single chain surfactants, protonated and non-protonated of oleyldimethylamine oxide, the dependence of beta on the degree of ionization (alpha) can be well described with a theoretical curve, derived on the basis of the linear relation between the surface charge density and the alpha value, despite the occurrence of the micelle-to-vesicle transition. (c) 2005 Elsevier Inc. All rights reserved.