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Fracture initiation sites were located in Mn-Mo-Nisteel specimens broken at different temperatures in the transition region. The stresses and strains at these sites were determined, and the failure probabilities calculated. The data shows that brittle fracture throughout most of the transition region can be characterised by defining a critical stress (J'c and a critical probability of failure at the position at which (J'c is achieved. This description implies that the measured toughness transition temperature can be expected to increase with a decrease in (J'CJ a decrease in the work to fracture, or an increase in yield stress. The variations in work to fracture with aging, proposed here to accommodate measured changes in toughness distributions, are consistent with the measured changes in phosphorus segregation with aging. The predicted proportionality between increase in yield stress and shift in transition temperature is similar to that found after irradiation.MST/3359