The effects of internal tritium and helium on the tensile properties of two austenitic stainless steels and an iron-based superalloy have been studied. The materials tested were, forged 21Cr-6Ni-9Mn and 304L (tested in the annealed condition and two forged conditions), and a modified A-286 alloy. The accumulation of 3He from the radioactive decay of tritium caused an increase in the yield strength and a continuous decrease in the ductility in almost all materials tested. Increased 3He concentrations also caused a change in fracture mode from ductile rupture to predominantly intergranular fracture. The property changes resulted from 3He bubble-induced strengthening, which produced a change in deformation mode from long-range dislocation activity to deformation twinning. In the deformation-twinning mode, the 3He-accelerated fracture initiated at the intersections of deformation twins with grain boundaries. High-strength forged 304L was most resistant to 3He effects, owing to the redistribution of 3He on dislocations.