Within the framework of the high-temperature gas-cooled reactor (HTGR) R&D agreement with GA Technologies, Inc., the Centre d’Etudes Nucléaires de Saclay investigated the transition temperature shift of the liner steel exposed to (thermal) neutrons. The steel was ferritic A537 (1.32% manganese, 0.26% copper, 0.26% silicon, 0.21% nickel, and 0.14% chromium). The specimens were irradiated in the French EL3 heavy water research reactor in an area where the neutron spectrum was comparable to that occurring in front of the HTGR core cavity liner:Φth/Φƒ ∼ 1000 . The temperature was 60 °C during the irradiations. For theoretical purposes, two irradiations were carried out at two different fluences. In addition, some specimens were cadmium plated to examine the effect of fast neutrons. Charpy impact tests were performed at Saclay with an instrumented impact device. The results show that current models overestimate the thermal neutron effect by a factor of 3.