Densities of aqueous solutions of mixtures of dodecyltrimethylammonium chloride (DTAC) and dodecyltrimethylammonium bromide (DTAB) have been measured as a function of total molality at constant composition and the apparent molar volumes of the mixtures were derived from the density data. The partial molar volumes of monomeric surfactant mixtures, the molar volumes of mixed micelles, and the volumes of formation of mixed micelles were evaluated and are compared with those for decyltrimethylammonium bromide (DeTAB) and DTAB mixtures. The partial molar volumes of monomeric surfactant mixtures and the molar volumes of mixed micelles are observed to depend linearly on the monomer and micelle compositions, respectively. Although the volume of formation of mixed micelles of the DeTAB-DTAB mixture depends on the micellar composition, that of the DTAC-DTAB mixture is observed to be almost independent of the micellar composition. This suggests that the volumes of the counter ions in the micellar solutions are almost equal to those in the monomeric solutions.