A continuum yield model for pressure dependent anisotropic materials that accounts for the stress state has been combined with a thermally activated model to account for the temperature and rate dependence of yield stress. The resulting model has been applied to describe the yield behavior of a semicrystalline aliphatic polyketone terpolymer. Utilizing biaxial data from this thermoplastic, best-fit values for loading along principal directions were determined. With these principal values, predictions for the yield strength under biaxial loading conditions were made. These predictions have been compared to experimental data obtained from the material at four different temperatures and three different strain rates. The experimental and theoretical results appear to compare quite well with each other.