The melting and crystallization behavior of polyethylene and isotactic polypropylene containing 1,2- or 1,3-disubstituted cyclopentane units in the main chain has been studied with simultaneous wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry. For the ethylene-based copolymers, the position of a reflection peak in the WAXD patterns shifts to a low angle with the increasing acquired temperature. The temperature dependence on the axial length of the crystal lattice is more marked in the copolymers forming orthorhombic crystals (containing 1,2-cyclopentane or 5.6 mol % 1,3-cyclopentane units) than in those forming hexagonal crystals (containing 8.1 mol % 1,3-cyclopentane units). For the isotactic propylene-based copolymers, the position of the reflection peaks in the WAXD patterns is independent of the acquired temperature. The proportion of the gamma form in the copolymer containing the 1,2-cyclopentane units is higher than that in the copolymers containing the 1,3-cyclopentane units. (C) 2004 Wiley Periodicals, Inc.