The yield behavior of an engineering thermoplastic under biaxial stress states has been investigated. The material considered is an aliphatic polyketone terpolymer. Multiaxial testing was performed on thin-walled hollow cylinders at four different temperatures and three strain rates. Various stress states were applied in order to develop failure envelopes. Within each envelope, the nominal strain rate along the octahedral shear plane, (gamma)over dot (oct), was held constant. These tests were performed at 0, 20, 50 and 80 degrees C at (gamma)over dot (oct) = 0.05 min(-1). At 20 degrees C, samples were also tested at (gamma)over dot (oct) = 0.005 and 0.5 min(-1). Below the T-g of 12 degrees C, failures in all stress states investigated, except axial compression, were brittle. At temperatures of 50 and 80 degrees C, all failures were ductile. At 20 degrees C, both ductile and brittle failures were observed. Although the rate affected the yield strength of the material, it had little effect on the mode of failure. In contrast, the temperature had a significant effect on the yield strength and mode of failure. While the effect of strain rate on yield strength was greater in the hoop direction than axial, the opposite was true for the effect of temperature. It was also observed that the state of stress played a significant role in the material failure.