Three series of isothermal torsion relaxation tests were performed at various temperatures ranging from room temperature to T = 120degreesC on isotactic polypropylene, low-density polyethylene, and linear low-density polyethylene. Constitutive equations are derived for the viscoelastic behavior of a semicrystalline polymer at small strains. A polymer is treated as an equivalent network of strands bridged by temporary junctions (entanglements, physical crosslinks on the surfaces of crystallites, and lamellar blocks). The network is thought of as an ensemble of mesoregions with various activation energies for detachment of active strands from their junctions. The time-dependent response of the ensemble reflects thermally induced rearrangement of strands (separation of active strands from temporary junctions and merging of dangling strands with the network). Stress-strain relations are developed by using the laws of thermodynamics. The governing equations involve five adjustable parameters that are found by fitting the experimental data. This study focuses on the influence of temperature and crystalline morphology of polyolefins on the material parameters in the constitutive relations. (C) 2004 Wiley Periodicals, Inc.