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Dopper broadening measurements of the gamma-ray spectra produced by positron annihilation have been made on an AISI 4340 (Unified Numbering System [UNS] G43400) steel as a function of both isochronal tempering over the temperature range of 150 to 700°C and isothermal tempering at 250°C for times ranging from 1 to 1100 min. Changes in the positron annihilation line shape parameter S are correlated with the corresponding structural transformations that accompany tempering of the as-quenched steel. Of particular significance is the increase in the S parameter in the temper embrittlement regime. This increase in S parameter is postulated to be related to the precipitation of a transition iron carbide that alters the defect structure of the surrounding ferrite phase.