J. D. Guo &etal; [J. Appl. Phys.80, 1623 (1996)] have studied the thermal stability of Ni on GaN and have deduced by x-ray diffraction measurements that new phases form at the interface between Ni and GaN. This determination, based solely on x-ray diffraction data, is ambiguous, as the peaks assigned to theGa4Ni3and Ni–N phases can also be attributed to substrate related x-ray peaks. ©1997 American Institute of Physics.