Supertough poly(butylene terephthalate) (PBT)-based blends were obtained by the melt blending of PBT with 0-30 wt % poly(ethylene-co-glycidyl methacrylate) (EGMA). The reaction between PBT and EGMA was detected by torque measurements. The particle size was almost constant with increasing EGMA content, and this indicated that compatibilization occurred. The minimum EGMA content for achieving supertoughness (i.e., an impact strength 16 times greater than that of PBT) was 20 wt %. The interparticle distance was the parameter controlling toughness in these PBT/EGMA blends. The dependence of the critical interparticle distance (tau(c),) on the modulus of the dispersed phase appeared only at low tau(c) values, and the primary dependence of tau(c) on the ratio of the modulus of the matrix to the modulus of the rubbery dispersed phase was proposed. (C) 2003 Wiley Periodicals, Inc.