Neutron diffraction has been used to determine the location and concentration of C in Nd2Fe17and in a Si substituted alloy. The results indicate that two carbide phases exist. One contains nearly three C atoms per formula unit, while the other has a much lower concentration. The high C phase contains C only on the 9esites, but the low C phase appears to contain C on the tetrahedral 18gsite. The changes in interatomic distances and cell volume are associated with the different site occupancies. ©1995 American Institute of Physics.