This paper presents and discusses a set of biaxial tests on two different polymer materials; PVC and HDPE. The biaxial tests are used to evaluate a hyperelastic–viscoplastic constitutive model involving the pressure-dependent Raghava yield criterion. The parameters in the model are calibrated from uniaxial tension and compression tests. Subsequently, the capability of the model is explored through nonlinear finite element analyses of the biaxial tests. The analyses are carried out using the material model implemented in LS–DYNA. A comparison of the force-displacement curves and the displacement fields from the simulations with those from the experiments shows that the model captures the main features of the behaviour observed in the tests.