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The distribution of divalent iron and nickel over two metal sites of differing coordination geometry in NiFe2(PO4)2, sarcopside, has been investigated by resonant X-ray and time-of-flight neutron powder diffraction. To assess the reproducibility of the X-ray technique, data have been collected from instruments X7A at Brookhaven National Laboratory and 8.3 at the Synchrotron Radiation Source, Daresbury Laboratory, England, using wavelengths λX1 = 1.7437 (3) Å and λX2 = 1.7434 (1) Å, respectively, close to the Fe2+ K edge determined by X-ray absorption near-edge structure. The real part of the anomalous-scattering correction for iron at each energy, f′(Fe)X1 = −7.81 (9) and f′(Fe)X2 = −10.16 (6), was determined experimentally by diffraction from Fe3(PO4)2 under identical conditions. Occupancies obtained for iron at the M(1) site were found to be M(1)X1 = 0.366 (6) and M(1)X2 = 0.376 (3), compared with M(1)N = 0.26 (15) from time-of-flight neutron powder diffraction.