Positron annihilation spectroscopy (PAS) based on positron lifetime (PL) measurements using the pulsed low-energy positron system (PLEPS) was applied for the investigation of defects of irradiated and thermally treated reactor pressure vessel (RPV) steels. PLEPS results showed that the changes in the micro structure of the RPV-steel properties caused by neutron-irradiation and post-irradiation heat treatment can be well detected. From the lifetime measurements in the near-surface region (20550 nm) the defect density in the Russian types of RPV-steels was calculated using the diffusion trapping model. The postirradiation heat treatment studies performed on non-irradiated specimens are also presented. (C) 2002 Elsevier Science B.V. All rights reserved.