The reduction of interfacial tension a between polyethylene oxide (PEO) and polypropylene oxide (PPO) by either a triblock copolymer EO-PO-EO or a diblock copolymer styrene-EO was studied by means of the pendant drop method from 70 to 120 degreesC. Time independent data correspond to stationary states and do not represent equilibrium information. The addition of the compatibilizer to the PPO phase is approximately twice as efficient as the addition to the PEO phase for both block copolymers. Knowing the concentration dependence of sigma for one of the coexisting phases it is possible to forecast the effects resulting from the addition of the other phase by means of model considerations based on the partition coefficient of the additive and the ratio of the viscosities of the coexisting phases. (C) 2001 Elsevier Science Ltd. All rights reserved.