Two poly(butylene terephthalate) (PBT)-rich compositions of PBT/poly(ethylene terephthalate) (PET) blends (5/1 and 2/1) were mixed with up to 30 wt% of a maleinized poly(ethylene-octene) copolymer (mPEO). The blends were composed of a PBT/PET amorphous phase, a pure mPEO amorphous phase, and the crystalline phases of PBT and mPEO. The average size of the dispersed mPEO particles was larger in the (PBT/PET 5/1)/mPEO blends than in the (PBT/PET 2/1)/mPEO blends, but in both cases larger than that of PET/mPEO blends and smaller than that of PBT/mPEO blends from previous works. The Young's modulus and the yield stress decreased with increasing mPEO contents and the ductility of the 2/1-based blends remained constant at higher mPEO contents compared to the 5/1-based blends. Super-toughness was obtained in both blends, but only the critical interparticle distance (IDc) of the (PBT/PET 5/1)/mPEO blends was used for comparison with those of PET/mPEO, poly(trimethylene terephthalate) (PTT)/mPEO and PBT/mPEO blends in terms of adhesion, because the (PBT/PET 2/1 1)/mPEO blends showed a simultaneous change in matrix modulus and adhesion. This comparison provided additional support to the proposed "saturation" of the interactions between the ester and maleic groups in this series and to the role of adhesion on IDc (higher adhesion, lower IDc). POLYM. ENG. SCI., 49:1013-1021, 2009. (C) 2009 Society of Plastics Engineers