Charpy impact tests were performed on martensitic/ferritic (MF) steels T91, F82H, Optifer-V and Optimax-A/-C irradiated in SINQ Target-3 up to 7.5 dpa and 500 appm He in a temperature range of 120-195 degrees C. Results demonstrate that for all the four kinds of steels, the ductile-to-brittle transition temperature (DBTT) increases with irradiation dose. The difference in the DBTT shifts (Delta DBTT) of the different steels is not significant after irradiation in the SINQ target. The Delta DBTT data from the previous small punch (Delta DBTTSP) and the present Charpy impact (Delta DBTTCVN) tests can be correlated with the expression: Delta DBTTSP = 0.4 Delta DBTTCVN. All the Delta DBTT data fall into a linear band when they are plotted versus helium concentration. The results indicate that helium effects on the embrittlement of MF steels are significant, particularly at higher concentrations. It suggests that MF steels may not be very suitable for applications at low temperatures in spallation irradiation environments where helium production is high. (c) 2005 Elsevier B.V. All rights reserved.