Positron lifetime, annihilation line shape, and electron microscopic measurements have been carried out on neutron irradiated pressure vessel steel (HSST Plate 03) meeting ASTM A533-B (Unified Numbering System [UNS] K12539). The positron annihilation parameters for specimens irradiated at 423 K to a dose of 1.5 × 10¹⁸ n/cm² are found to decrease sharply during the post-irradiation annealing interval, 473 to 623 K. This correlates well with the stage seen in the recovery of Vickers hardness. On the basis of known evidence for carbon migration in α-iron and the interpretation of present results, enhancement and recovery of radiation hardening have been qualitatively understood in terms of formation and dissolution of carbon decorated vacancy complexes.