The influence of the chemical structure of the hydrophilic group on the miscibilities of binary cationic surfactants in the adsorbed film and micelle was investigated by measuring the surface tension of the aqueous solution of N-methylated dodecylammonium chloride and dodecylammonium chloride (DAC) mixtures as a function of the total concentration and composition of surfactants. It was found from the shapes of the phase diagrams of adsorption and micelle formation that dodecylmethylammonium chloride (DMAC) mixes ideally with DAC in the adsorbed film and micelle and that dodecyltrimethylammonium chloride (DTAC) does so nonideally. Furthermore, by comparison of the compositions of the adsorbed film and micelle at the critical micelle concentration (CMC), it was confirmed that larger surfactant hydrophilic groups are geometrically favorable for the formation of the spherical micelle. It was also indicated that the surface tension at the CMC of the surfactant mixture provides useful information when it is plotted against the compositions of the adsorbed film and micelle.