Following the Adam-Gibbs procedure for describing the relaxation process of glass-forming liquids, a Vogel-Fulcher type equation has been obtained, determined by a new structural parameter called the isovolume fictive temperature, T(x). The resulting expression for the relaxation time has been applied satisfactorily to the experimental data of dielectric relaxation during physical ageing of poly(vinyl acetate). For testing the reliability of the proposed model, comparison with other phenomenological approaches has been made.