AbstractSamples of the nickel base alloy Nimonic 86 were exposed to PNP‐500 type helium for 1000 h at 650, 750, 850, 900 and 950°C. X‐ray diffraction showed 5 additional phases, formed at the sample surfaces during the heat treatment. They were shown to be: (1) a spinel Cr2MnO4(at T ≤ 850°C) containing increasing amounts of Al and Ti with increasing temperature, (2) a sesquioxide Cr2O3(at T ≥ 900°C) containing substantial amounts of Mn for T ≤ 850°C, (3) an ≠1oxide Ni3Mo3SO (mainly at 750°C) containing some Fe, (4) an ≠1carbide (Ni, Cr, Si)3Mo3C (at T ≤ 750°C), (5) Cr23C6(at T ≤ 85