Micellization of seven binary surfactant mixtures (containing an odd carbon chain nonionic surfactant, tridecanol ethoxylates (20 mol of ethylene oxide/mol of tridecanol), represented as C13E20, and a nonionic or cationic surfactant) was studied by dye absorption method. Critical micelle concentrations (CMCs) of these mixed surfactant systems in water were obtained spectrophotometrically using Eosin Y dye. A pseudo-phase separation model was employed for mixed micellization analysis. The Margules equation with one constant was employed to describe nonideal mixing behavior in micelles of surfactant mixtures. The nature of nonideality is accounted for by intermolecular interactions and micellar composition. The composition of mixed micelle x(1) and x(2) and the interaction parameter, were determined from the CMCs of different binary systems, employing the Margules equation. Mixtures of nonionic surfactants show antigistic interactions while cationic-nonionic surfactant mixtures show strong synergistic interactions.