Micelle formation was studied for mixed surfactant systems of a nonionic surfactant, nonanoyl-N-methylglucamine (MEGA-10) with a bile salt, sodium deoxycholate (NaDC), in water at 30 degrees C. NaDC and MEGA-10 are quite different from each other in terms not only of hydrophobic but also of hydrophilic groups. Surface tension of the aqueous solution (gamma) at different mixing ratios was measured by the drop volume method, and critical micellization concentrations (CMCs) were determined from the gamma vs CMC curves at discrete mole fractions of MEGA-10 (X-MEGA) in the surfactant mixture. The CMC-X,,, curve showed a positive deviation from ideal mixing, The apparent interaction parameter (W-R) upon micelle formation was evaluated by applying the Rubingh equations derived on the basis of the regular solution theory. The different theory developed by Motomura et al, has been also applied. The analysis of CMC data indicated that the properties of mixed micelles are different between the ranges below and above X-MEGA congruent to 0.2 where an azeotrope exists and that the composition of mixed micelles is very similar to that of monomeric species in bulk solution.