The composition of the micelles in binary mixtures of the cationic amphiphilic drugs (a) chlorpromazine and promethazine and (b) chlorpromazine and adiphenine has been determined from an analysis of the variation of the critical micelle concentration, as measured by conductivity techniques, as a function of solution composition. Evaluation of the nonideality of mixing of the components of the mixed micelles using a regular solution approximation revealed ideality of mixing in the chlorpromazine I promethazine system. Mixing in the chlorpromazine/adiphenine system did not deviate significantly from ideality despite differences in the structure and packing characteristics of these two drugs.