Abstract Torsion and normal force measurements were made during single step stress relaxation experiments on a polymeric glass (PMMA). Isochronal data were analysed using an approach adapted from that developed by Penn and Kearsley 1 (for incompressible elastic materials) to determine the derivatives ∂W ∂I 1 , and ∂W ∂I 2 of the time dependent strain potential function. ∂W ∂I 1 and ∂W ∂I 2 are determined from existing solution to the torsion of an incompressible cylinder. A special solution to the torsion of a compressible cylinder is presented and it is shown that the values of ∂W ∂I 1 and ∂W ∂I 2 obtained using this solution to analyse the data do not differ greatly from those obtained using the incompressible solution. It is found from both solutions that ∂W ∂I 1 is negative and increases towards zero with increasing time and deformation while ∂W ∂I 2 is positive, greater in magnitude than ∂W ∂I 1 and decreases towards zero with increasing time and deformation. These results were unexpected and a full understanding of their meaning has yet to be reached.