Abstract: Environmental embrittlement in A3B-type intermetallics based on Ni3Al and Fe3Al has been studied in this paper. For the Ni3Al doped with 120 wt ppm B and Ni,(Al,Cr.Zr) doped with 80 wt ppm B,their elongation and ultimate tensile strength decreased in the sequence:of vacuum > air >hydrogen. while for Ni,(Al,Mn) doped with 400 wt ppm B no envifonmental degradation was ob served, although a -Ni3(Al,Mn) alloy without B showed a decrease in ductility when tested in air in stead of oxygen. It is supposed that boron and hydrogen compete for the occupation of interstitial sites near grain boundaries. If boron content is sufficiently low, hydrogen embrittlement occurs ;however, if its content is sufficiently high. boron addition is capable of eliminating envjronmental ef fect in Ni3Al-based alloysi As to the micromechanism of hydrogen embrittlement in Ni3Al+B. S EM in situ observations showed that both grain boundary decohesion and a high stress concentration con tributed to hydrogen-assisted jntergranu lar cracking in this alloy. For the Fe3Al and Fe3 (Al.Cr) alloys.their mechanical properties depended strongly on grain size / grain shape and testing environment. A strain rate effect on ductiIity and fracture strength was also observed in the Fe3Al and Fe,(Al,Cr)+B aIloys. Preoxidation increased the ductility of the Fe,(Al,Cr)+B alloy. All these results can be rationalized from a hypothesis that surface reaction is the controlling process in embrittling Fe3Al-based alloys.