The Feynman path integral quantum Monte Carlo formalism has been combined with an electronic Hamiltonian of the B3LYP-EPRIII type to study the isotropic hyperfine coupling (hfs) constants of the ethyl radical in thermal equilibrium (T = 25 K). The nuclear vibrations and the intramolecular rotation around the C-C bond have a sizeable influence on the hfs parameters. The thermally averaged numbers differ from single-nuclear configuration values calculated at the minimum of the potential energy surface. The analysis of the distribution functions for the isotropic hfs constants Of C2H5 offers insight into the strong coupling between calculated hfs parameters and the nuclear configuration adopted. (C) 2004 Elsevier B.V. All rights reserved.