Abstract The combined effects of both silicon and helium on void nucleation in a high purity stainless steel have been studied during electron irradiation in a high voltage electron microscope in the temperature range 400–750°C. The nucleation of voids was not affected by silicon up to 0.5wt%, either with or without 10ppm helium. At higher silicon levels continuous nucleation occurred with a reduced maximum void density. The void density was increased by helium in all case.s and up to 0.5% silicon showed a characteristic two stage temperature dependence, with activation energies of 0.5 and 1.0eV, in contrast to the single stage in alloys without helium and in >0.5% silicon materials with helium. The results are discussed in terms of two mechanisms; the binding of gas to silicon atom clusters and the segregation of silicon to void nuclei.