The fracture resistance and micromechanisms of two tempered martensitic steels were characterized over a range of temperatures by both mechanical testing and quantitative fractography. HT-9 and F-82H undergo a fracture mode transition from quasi-cleavage at low temperature to microvoid coalescence at high temperature. The transition in HT-9 is rather gradual, and the transition in F-82H is extremely abrupt. While the toughness of F-82H was higher in all cases, differences between the two steels depended on test type, strain rate and temperature. The results not only demonstrate the inherent non-uniqueness of the so called ductile-to-brittle transition temperature (DBTT), but also call into question the use of Charpy data for ranking the relative performance of various alloys. The similarities in quasi-cleavage fracture and differences in ductile fracture are manifested in the fracture surfaces and are interpreted in terms of the underlying mechanisms and microstructures.