Mixtures of zwitterionic and cationic surfactants were studied in the presence of an anionic polyelectrolyte, We used N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB12), dodecyltrimethyl ammonium bromide (DTA(+)Br(-)), and polyvinyl sulfate (potassium salt) (PVS-K+), The composition of the system was determined by measuring the activity of DTABr with an ion-selective electrode, and deducing the activity of SB12 from thermodynamic equations. This method provides information about the distribution of all species (activities of both surfactants and composition of the micelles), by experimental determination of only one quantity. SB12 affected the adsorption of DTABr onto PVSK.