Simple blends with different viscosity ratios of the components as well as compatibilized blends varying both in type and content of the compatibilizers were used to study the relation between the interfacial tension and the dispersed-phase particle size for PP/PA6 (80/20 wt %) blends in this work. Four compatibilizing systems including poly(ethylene-co-methacrylic acid) ionomers, a maleic anhydride-grafted propylene copolymer, maleic anhydride-grafted polypropylene, and a maleic anhydride-grafted styrene ethylene butylene copolymer were used. For blends prepared in an internal mixer, a power-law relation was found between the capillary number and the torque ratio of the blends' components. This relation was used to estimate the interfacial tension for the compatibilized blends. The relation between the steady-state torque of the blends as a measure of viscosity and the estimated values of interfacial ension were also investigated. (C) 2003 Wiley Periodicals, Inc.