A series of solutions of constant composition of the mixed micelles but different in total concentration of surfactants were prepared with the help of the Clint theory for mixed micelles consisting of octanonyl-N-methylglucamide (Mega-8) and nonanoyl-N-methylglucamide (Mega-9), and the partial molar volumes V-2 were measured. The result demonstrated that the conventional procedure for investigation of the composition dependence of physical properties like V-2 of mixed micelles is acceptable for the systems containing homologous surfactants. Then, the composition dependence of the partial molar volume V-2 and the partial molar adiabatic compressibility K-s degrees were measured for the cationic mixed micelles decyltrimethylammonium bromide (C(10)TAB) and dodecyltrimethylammonium bromide (C(12)TAB) at 25 degrees C. In both mixed micelle systems K-s degrees of the micelle showed deviation from the linear relation with composition, while V-2 varied linearly with the composition of the micelles. The dimension of the mixed micelles of Mega-8 and Mega-9 was also estimated from the diffusion coefficients, which showed much larger values than that of V-2.