Oversized solute additions (0.3 at% of either Pt or Hf) were found to have a significant effect on microstructural and microchemical evolution in ultra high-purity 316 stainless steel model alloys after proton irradiations at 400°C to nominal doses of 2.5 and 5 dpa. The radiation-induced microstructure of the 316SS base alloy consisted of voids and faulted Frank loops at both dose levels. Platinum addition enhanced void nucleation and increased the loop density and the mean loop diameter. Hafnium addition to 316SS delayed void formation, decreased the mean loop diameter and increased loop density. Grain boundary composition was measured using analytical transmission electron microscopy. Strong radiation-induced grain boundary Cr depletion and Ni enrichment was found in the 316SS base alloy, while oversized element additions reduced segregation levels at low dose. The Pt-doped alloy showed only minor radiation-induced changes in grain boundary composition and no evidence of segregation was observed in the Hf-doped alloy at 2.5 dpa, however, increasing dose resulted in significant radiation-induced segregation in the Hf-doped alloy.