Blends of poly(hydroxy ester ether) (PHEE), a recently developed bisphenol A ether-based synthetic biodegradable thermoplastic polymer, with a soybean protein isolate and two hydrolyzed wheat glutens were studied. Blends of the proteins with PHEE were produced from 20 to 70% by weight of protein content. Young's moduli of the protein/PHEE blends fall in the range of 0.8-1.5 GPa with tensile strengths ranging from 10 to 30 MPa. Critical stress-intensity factors of the blends ranged from 2 to 9 MPa-m(1/2) depending on the amount of protein added. Morphological analysis indicated a moderate degree of adhesion between the protein and PHEE phases in the blends. In general, as the protein content was increased the materials lost ductility and failed in a brittle manner; however, the mechanical properties of several compositions were comparable to commercial thermoplastics such as polystyrene.