The partition coefficients of 1-alkanols between water and didodecyldimethylammonium bromide (DDAB) vesicle membrane, and the change of the degree of counter-ion dissociation on DDAB vesicle caused by the solubilization of 1-alkanols were determined from electric conductivity measurements. A good linear relationship was obtained between logarithmic value of the partition coefficient and carbon number of alkyl chain in 1-alkanols. The standard transfer free energy change per methylene group was evaluated from the slope of this straight line. On the other hand, the effect of 1-alkanols on the degree of counter-ion dissociation was independent of carbon number of 1-alkanols. Comparing the results of the present system with those of sodium dodecylsulfate (SDS) micelle/1-alkanol system, it was found that the interior of DDAB vesicle membrane is more hydrophobic than that of SDS micelles. Furthermore, the increase in counter-ion dissociation caused by solubilized 1-alkanols was much lesser for DDAB vesicles than SDS micelles.