Abstract: Impact toughness tests, low frequency internal friction measurements, TEM and SEM structural analyses were carried out on a series of medium carbon, low alloy steels. Experimental results indicate that the high temperature (reversible) temper embrittlement is considered as the result of the aging effect ofα-solid solution causing dead-pinning of dislocations by precipitated ultrafine Fe3C (N) particles. The beneficial effect of molybde aum in diminishing this embrittlement lies in the prevention of the aging processes by its strong affinity to C (N) atoms.