Rietveld analysis of neutron powder diffraction data has been used to investigate the compositions of XNiSn (X = Ti, Zr, Hf) half-Heusler alloys prepared by solid state reactions. All samples containing Ti have 2-3% excess Ni, whereas the samples with X = Zr, Hf are almost stoichiometric.Samples with mixed X-metals are characterised by the presence of 3-4 distinct X 1-x X' x Ni 1+y Sn half-Heusler phases. Variable temperature and time dependent neutron powder diffraction for X = Ti and X = Ti 0.5 Hf 0.5 demonstrates that both the amount of excess Ni and the phase distribution are stable up to at least 600 °C. Debye temperatures of 367(2) K and 317 (2) K were obtained from the thermal displacement parameters. The samples containing Ti are characterised by a ~0.15 eV bandgap and a monotonously decreasing Seebeck coefficient. The compositions with Zr and Hf have similar bandgap values but show ambipolar transitions. Analysis of the thermoelectric transport data of degenerately doped Ti 0.5 Zr 0.5 NiSn 1-z Sb z samples using a single parabolic band model demonstrates that the transport is limited by alloy scattering and yielded an effective carrier mass of 2.5(1) m e .