Thermal ageing for up to 1000 h at 600-800 degrees C was used to study the long-range ordering behaviour to Ni4Mo and related ordered phases in selected Ni-Mo based alloys with varying Fe and Or contents, and the corresponding effects on mechanical properties. Analytical electron microscopy and X-ray diffraction were used to characterize the microstructure. Mechanical properties were determined from microhardness and tensile tests. During the early stages of ordering, the crystallographically-related Pt2Mo-type, DO22 and D-1a superlattices coexisted in all alloys studied. However, depending upon the exact chemical composition, particularly the Mo, Cr and Fe contents, some of these superlattices became unstable as the ordering reaction progressed. Chromium was found to act as a stabilizer of either a Pt2Mo-type superlattice or Ni3Mo depending upon the Mo content; however Fe acted as a stabilizer only of Ni3Mo. For both Cr and Fe, the tendency to stabilize Ni3Mo was realized at relatively higher Mo contents. Ordering behaviour of commercial alloys containing minor concentrations of Or and Fe was found to significantly vary from one heat to another depending upon the exact Mo content. Although ordering to Ni4Mo and Ni3Mo could lead to severe embrittlement, ordering to a Pt2Mo-type superlattice was found to have beneficial effects on mechanical strength.