The effects of small ions ( z + z − electrolytes) and long-chain alkyl ions (ionic surfactants) on the surface potential ψ 0 of the micelle of a cationic-nonionic surfactant, dimethyldodecylamine oxide, have been studied by potentiometric titrations of its micellar solutions. The effect of small ions on ψ 0 is in accord with the effect predicted from the Gotüy-Chapman equation for spherical particles. Small counter-ions, especially their valences, have considerable influence on ψ 0 whereas co-ions have no remarkable influence. The effect of long-chain alkyl ions on ψ 0 is directly related to the charge properties of the added surfactant, probably owing to the formation of mixed micelles with the amine oxide surfactant.