A 0.45 C, 9 Ni, 4 Co alloy steel is being used to study the effect of thermomechanical treatments on high strength steels with particular interest in fracture toughness. It was found that austenite deformation narrows the metastable austenite bay region with respect to both transformation time and temperature. The A sub 1 temperature of this steel was found to be approximately 1225 F. Fifty percent austenite deformation lowers the Ms temperature by 10 F, but does not significantly alter the range of temperatures over which the bainite reaction occurs. Electron and X-ray diffraction techniques have been employed to locate the various stages of tempering. A marked increase in fracture toughness occurs over a narrow tempering temperature interval as the carbide in the martensite changes from epsilonto cementite. Subcooling of this alloy is necessary in order to reduce the retained austenite content which has a substantial effect on the mechanical properties. Straintempering treatments improve the strength of martensitic and bainitic structures but decrease the fracture toughness.