Two polyethylene/polypropylene/poly(ethylene-co-propylene) in-reactor alloy samples with a good polymer particle morphology were synthesized by sequential multistage gas-phase polymerization with a spherical Ziegler-Natta catalyst. The alloys showed excellent mechanical properties, including both toughness and stiffness. With temperature-gradient extraction fractionation, both alloys were fractionated into five fractions. The chain structures of the fractions were studied with Fourier transform infrared, (CNMR)-C-13, and thermal analysis. The alloys were mainly composed of polyethylene, polyethylene-b-polypropylene block copolymer, and polypropylene. There also were minor amounts of an ethylene-propylene segmented copolymer with very low crystallinity and an ethylene-propylene random copolymer. The block copolymer fraction accounted for more than 44 wt % of the alloys. The coexistence of these components with different structures was apparently the key factor resulting in the excellent toughness-stiffness balance of the materials. (c) 2005 Wiley Periodicals, Inc.