The change in the critical micelle concentrations (CMC) of alkylammonium chlorides (C12–C18) and in the degree of the dissociation of the micelle (α) of dodecylammonium chloride was determined by adding 1-propanol and acetone to water. Both the CMC and the α increased with an additive concentration, each reaching a maximum at a certain concentration (critical composition of solvent for micelle formation, CCSM) and decreasing beyond the CCSM. The increment in the CMC was more influenced by the alkyl-chain length in the surfactant molecule in the presence of 1-propanol than in that of acetone. The effective coefficient of electrical energy (Kg) and the cohesive energy difference per methylene group between the micelle and the monomer states (ω) were calculated from the CMC and the α. The Kg values in both mixed solvents were constant beyond the respective CCSM. The value of ω was less in the presence of 1-propanol than in that of acetone.