The influence of chain length on the composition of mixed micelles in binary mixtures of N,N-dimethyldodecan-1-amine oxide (DDAO) and decyl-, dodecyl-, and tetradecyl-trimethyl-ammonium bromide surfactants (abbreviated as C(10)TAB, C(12)TAB, and C(14)TAB, respectively) has been determined at 298 K. From the surface tension measurements, the critical micelle concentration (cmc) data were measured as a function of mixing composition. The cmc values were then analyzed according to regular solution model. The composition of mixed micelles, the interaction parameter (beta), and the activity coefficients were evaluated from the regular solution model for all cationic-amphoteric mixed systems. The interaction parameter values indicated somehow a small deviation from ideality for the three mixed surfactant systems, for all investigated mole fractions. The strength of the interaction between the amphoteric surfactant and the alkyl-trimethyl-ammonium bromide surfactants in three mixed systems obeys the following order: (C(10)TAB + DDAO) > (C(12)TAB + DDAO) > (C(14)TAB + DDAO) suggesting that the decrease in the length of cationic surfactant alkyl chain results in a stronger interaction with DDAO. Rheological measurements of these solutions in the presence of sodium salicylate, NaSal, for the three mixed systems were also measured. The viscosity data reveal a viscosity increasing trend of (C(14)TAB + DDAO) > (C(12)TAB + DDAO) > (C(10)TAB + DDAO).