The effects of neutron irradiation on the steel reactor vessel for the modular high-temperature gas-cooled reactor (MHTGR) are eing investigated, primarily because the operating temperatures are low [121 to 288°C (250–550°F)] compared to those for commercial light-water reactors (LWRs) [∼288°C (550°F)]. The need for design data on the reference temperature (RTNDT) shift necessitated the irradiation at different temperatures of A 533 grade B class 1 plates, A 508 class 3 forging, and welds used for the vessel shell, vessel closure head, and vessel flange. This paper presents regular- and mini-tensile, Automated Ball Indentation (ABI), and Charpy V-notch (CVN) impact test results from five irradiation capsules of this program. The first four capsules were irradiated in the University of Buffalo Reactor (UBR) to an effective fast fluence of 1.1 × 1018 neutrons/cm2 [0.7 × 1018 neutrons/cm2 (>1 MeV)] at temperatures of 288, 204, 163, and 121°C (550, 400, 325, and 250°F), respectively. The fifth capsule (designated ORNL-7) was irradiated in the Ford Nuclear Reactor (FNR) of the University of Michigan at 60°C (140°F) to an effective fast fluence of 1.3 × 1018 neutrons/cm2 [0.8 × 1018 neutrons/cm2 (>1 MeV)]. The yield and ultimate strengths of both A 533 grade B class 1 plate materials of the MHTGR program increased with decreasing irradiation temperature. Similarly, the 41-J CVN transition temperature shift increased with decreasing irradiation temperature (in agreement with the increase in yield strength). The mini-tensile and Automated Ball Indentation (ABI) test results (yield strength and flow properties) were in good agreement with those from standard tensile specimens. The mini-tensile and ABI test results were also used in a model which utilizes the changes in yield strength to estimate the CVN ductile-to-brittle transition temperature shift due to irradiation. The model predictions were compared with CVN test results obtained here and in earlier work.