Mixed micellization properties of the cationic monomeric surfactants hexadecyltrimethylammonium bromide (CTAB), hexadecyltriphenylphosphonium bromide (CTPB), hexadecyltributylphosphonium bromide (CT-BuPB), and tetradecyltriphenylphosphonium bromide (TTPB) with gemini surfactant C(16)H(33)(CH(2))(2)N(+)-(CH(2))(10)N(+)(CH(2))(2)C(16)H(33)center dot 2Br(-) (C(16)-10-C(16)center dot 2Br(-)) have been investigated by performing conductivity measurements in aqueous solution. The conductance data were used to obtain the values of the critical micelle concentration (cmc) of mixed surfactant systems having different compositions. The mixed cmc values determined from the experimental data were used to calculate the interaction parameter (beta) using regular solution theory for the mixed surfactant system. The erne values show nonideality, and the negative values of beta indicate an overall attractive force in the mixed state. Also, the measured values of the excess free energy of mixing have negative values for all of the systems.