The densities of aqueous solutions of a decylammonium chloride (DeAC)-dibucaine hydrochloride (DC HCl) mixture and a DeAC-tetracaine hydrochloride (TC HCl) mixture were measured as a function of the total molality and composition of the mixture at 298.15 K under atmospheric pressure. The molar volumes of the mixture in the monomeric and micellar states of both systems were evaluated by applying the thermodynamic equations to the apparent molar volumes obtained from the density data. They varied linearly with composition between the values of pure components. The volume of mixed micelle formation also showed the Linear dependence on the composition. These facts indicate that the DeAC and the anesthetic molecules mix ideally in both the monomeric and micellar states from the viewpoint of volume. On the other hand, the micellar composition was calculated from the composition dependence of the critical micelle concentration values. The shape of the phase diagram of micelle formation was slightly distorted cigar-type due to the nonideal interaction between DeAC and the anesthetic molecules in the mixed micelle. Ideal mixing of volume in the micellar state indicates that the nonideal interaction is not so strong that it affects the molar volume of the micelle.