The effect of reactive interfacial agents on the interfacial tension of nylon-6/polystyrene (PS) blends was studied. Two types of Li end-sulfonated polystyrene (e-SPS), namely (omega-SPS and alpha, omega-SPS) were used as the reactive interfacial agents. The interfacial tension in the melt state was measured by the breaking-thread method. As the e-SPS content was increased, the interfacial tension of the nylon-6/PS blend decreased and reached a saturation value, as observed in the case of block copolymers. An effect of the structure of e-SPS on the interfacial tension was observed : for omega-SPS, the saturation value of the sulfonate group concentration was very low (2 x 10(-5) moll(-1)) and no molecular-weight effect was observed; however, for alpha, omega-SPS, the saturated interfacial tension was attained at a relatively higher sulfonate group concentration than that of omega-SPS,while the lower molecular weight of alpha, omega-SPS reduced the saturated interfacial tension to a lower value.