The critical micelle concentration $(CMC^{ast})$ and the counterion binding constant (B) in a micellar state of the mixed surfactant systems of dodecylpyridinium chloride (DPC) with dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), and cetyldimethylethylammonium bromide (CDEAB) were determined at $25^{circ}C$ as a function of ${alpha}_1$ (the overall molc fraction of DPC) by the use of electric conductivity method. Various thermodynamic parameters $(X_{i},;{gamma}_{I},;C_{i},;a^{M}_{i}, eta,;{Delta}H_{mix}, ;and; {Delta}G^{o}_{m})$ for the micellization of DPC/DTAB, DPC/TTAB, and DPC/CDEAB mixtures were calculated and analyzed for each system by means of the equations derived from the pseudophase separation model. The results show that the DPC/DTAB mixed system has the greatest deviation and the DPC/CDEAB mixed system has the smallest deviation from the ideal mixed micellar model.