We investigated the interfacial tension gamma between poly(ethylene oxide) (PEO) and polystyrene (PS) focusing our attention on the dependence of gamma on the molecular weight (M) of PEO and the surfactant effect of poly(ethylene oxide-b-styrene) diblock copolymer [P(EO-b-S)]. Measurements of gamma were carried out by observing electric deformation of droplets of PEO suspended in bulk PS or in a concentrated solution of PS in dioctylphthalate (DOP). The results indicate that gamma between PEO and PS is almost independent of M of PEO in the high molecular weight region, but exhibits a minimum around M = 500. Addition of 1 wt% of P(EO-b-S) to the PEO/PS system causes a decrease of gamma due to the surfactant effect of the copolymer, but gamma is almost independent of further addition of the copolymer. The decrement of the interfacial tension Delta gamma increases with increasing molecular weight of P(EO-b-S). (c) 2006 Elsevier Ltd. All rights reserved.