Cross-differentiation relations are used in thermodynamics to describe the behavior of cationic and nonionic surfactants in mixed micelles. This study uses dodecyltrimethylammonium bromide (DTABr) as cationic surfactant and dodecylpoly(oxyethylene glycol ether)(23), or BRIJ 35, as nonionic surfactant. One of these cross-differentiation relations allows us to connect the nonionic surfactant activity with that of the cationic surfactant by following the variations of DTABr molality with BRIJ 35 molality, at constant DTABr chemical potential. This was possible with a reliable DTA(+)-selective electrode. We show that this approach gives access to the activity of nonionic species in solution and into the micelle from potentiometric measurements.