[en] In fusion reactor environment, transmutation reactions occur that produce helium when neutrons from the plasma strike the containment first wall. This dissolved helium in the metal together with the neutron induced damage leads to premature, intergranular failure. In an attempt to clarify the role of helium in promoting high temperature intergranular failure, in situ tensile experiments were conducted in a high voltage electron microscope (HVEM) attached to a linear ion accelerator used at 80 keV. Two types of experiments were performed on 14-MeV neutron and helium irradiated 40-μm thick foil stainless steel specimens using a heating quantitative load elongation tensile stage between 25 and 700⁰C. A graph of cumulative crack length plotted against total specimen elongation exhibits a slope, S, which increases as specimen ductility (reduction of area, ROA) decreases. It was found that ROA² x S = 0.4 for all cases studied. High temperature helium embrittlement occurs only at slow strain rates. Dislocations dragging helium bubbles to grain boundaries is commonly thought to cause the embrittlement. Specimens tensile tested under dual tensile test temperatures exhibited integranular failure at 700⁰C and after the tensile test was resumed at 550⁰C charged to a transgranular failure mode