Tensile properties have been investigated on the low activation F82H ferritic/martensitic steels for both unirradiated and irradiated material. The irradiations have been carried out with protons of medium energy (590 MeV) at the PIREX facility and doses from 0.2 up to 1.75 dpa have been reached for temperature ranging between 310 and 673 K. The irradiation induced hardening has been systematically measured at room temperature for the different irradiation conditions (dose and irradiation temperature). The tensile tests have been supplemented with stress relaxation experiments in order to determine the activation volume of the rate controlling mechanism for dislocation motion. The effects of irradiation on the tensile properties and activation volumes are presented. (C) 1998 Elsevier Science B.V. All rights reserved.