The irradiation-induced behaviour of iron–chromium model alloys and high chromium ferritic/martensitic steels have been studied using positron annihilation measurements. Both the positron lifetime and coincidence Doppler broadening techniques were used in a complementary way. It is shown that the presence of chromium in iron-based alloys significantly reduces the concentration of vacancies to the extent that the formation of clusters is hindered. The observed vacancy behaviour as a function of the chromium content is fully consistent with the findings of void swelling suppression in chromium-rich alloys and can be rationalized based on existing models. Finally, the information obtained for the ferritic/martensitic steels shows good agreement with the results for binary model alloys. Compared with the vacancy behaviour in pure iron, this confirms the major influence of chromium on the nanostructural evolution of these steels under irradiation.