The binding of tetradecylpyridinium chloride (TPC), decylammonium chloride (DAC), and their mixtures to sodium poly(L-glutamate) (P(Glu)) were studied by means of a potentiometric titration method and circular dichroism. In the polymer-free solutions, the electrode exhibited the Nernstian response even for the binary surfactants. In the presence of the polymer, cooperative binding occurred in each system. Binding isotherms of TPC component were constructed, assuming that the membrane responds only to TPC in the mixed surfactant systems according to the electrode selectivity. We found a sort of synergetic effect in the interaction of the mixed surfactants to polymer. An ideal mixing model was derived by applying a phase-separation approach to the equilibrium between single dispersed surfactant in the bulk and bound surfactant in the polymer domains at the same electrostatic potential. The model was used successfully to predict the critical aggregation concentrations of the binding and the binding isotherms as well, Finally, conformational change of P(Glu) in surfactant solution was measured by circular dichroism. The alpha-helical conformation of P(Glu) was induced only in the pure DAC solution, In the binary-surfactant solution, the transition to the ordered conformation was inhibited by the ideal mixing of TPC and DAC bound to P(Glu), presumably due to the steric hindrance introduced by the bulky headgroup of the TPC surfactant.