The volumetric mixing in anionic/nonionic, cationic/nonionic, and anionic/cationic mixed micelles was determined by examining the total surfactant apparent molar volumes (phi(upsilon,tot)) at total surfactant concentrations (C-T) much greater than the mixture critical micelle concentration (CMC(m)). The mixed surfactant systems investigated were sodium dodecyl sulfate and a polyethoxylated nonylphenol, at 0.15 M NaCl and with no added NaCl; cetyl pyridinium chloride and polyethoxylated nonylphenol at 0.03 M NaCl; and sodium dodecyl sulfate and dodecyl pyridinium chloride, at 0.15 M NaCl. For all of the mixed surfactant systems investigated, the volumetric mixing in the mixed micelles at C-T much greater than CMC(m) was ideal, even though the free energies of mixing at the onset of mixed micelle formation (as indicated by the mixture critical micelle concentrations at C-T = CMC(m)) exhibited strong negative deviations from ideality. Furthermore, added electrolyte was shown to have virtually no effect on the phi(upsilon,tot) data (nor, consequently, the volumetric mixing) for the anionic/nonionic mixed surfactant system. The results of this study suggest that at C-T much greater than CMC(m), the electrostatic interactions do not significantly affect the molar volume of the mixed micelle. Therefore, the micelle hydrophobic core dominates the volumetric mixing in mixed micelles at CT much greater than CMC(m).