The interaction between sodium dextran sulfate (DxS) and alkyltrimethylammonium bromides was studied by means of potentiometric titration and electrophoretic light scattering methods in the phosphate buffer of pH 7.0 and ionic strength 0.014 at 25 degrees C. The bindings of dodecyltrimethylammonium ion and tetradecyltrimethylammonium ion to DxS occurred in two stages. In the first stage, the steep rise of the binding number is characteristic of a strong cooperative binding. The profile of hexadecyltrimethylammonium ion binding was similar to those of the others, as a whole. However, the concentration region, where the binding proceeded, became lower with an elongation of the hydrocarbon chain of the surfactant. The mobility of DxS was -3.7 x 10(-4) cm(2) s(-1) V-1 in the absence of the surfactant. In the first binding stage, the negative mobility of DxS decreased with an increase in each surfactant concentration. It crossed zero mobility when the binding degree of the surfactant (moles of bound surfactant ion/mole of sulfate group of DxS) was 1. In the second stage where the binding degree exceeds 1 and approaches 2, a further binding of the surfactant ions appears to form micelle-like aggregates on DxS. The mobility of DxS became positive because of excessive positive charges introduced on the polymer. The mechanism of the complex formation is considered to consist of not only the binding process of the surfactant but also a change of shape of the complex itself.