The change in the contact angle and the interfacial tension (gamma) between poly(butylene terephthalate) (PBT) and polystyrene (PS) depending upon the amount of an in situ compatibilizer poly(styrene-co-glycidyl methacrylate) (PS-GMA) is investigated by the Neumann Triangle method using the third component of poly(methyl methacrylate) (PMMA). With increasing amount of PS-GMA in the total PS phase. gamma sharply decreased at small amounts of PS-GMA followed by a gradual decrease. We have shown that the optimal experimental conditions for minimizing any possible error in determining contact angles, thus gamma, are that: (i) both the upper and the lower parts in the middle phase should be parts of their own sphere; and (ii) the exact half of the middle phase in a specimen should be perpendicularly cut (or polished). The changes in contact angles due to any deviation from the optimal conditions are analyzed and compared with experimental results. (C) 2001 Published by Elsevier Science Ltd.