Nine (Fe1−xMnx)3(PO4)2 solid solutions (0.1≤x≤0.9) with the graftonite-type structure have been prepared and equilibrated at 1070 K. The structure contains three distinct cation coordination polyhedra, all distorted: one octahedron, and two five-coordinated polyhedra. Accurate unit cell dimensions (P21/c) have been obtained from Guinier–Hägg photographic data for these phases. Mossbauer spectra in combination with a neutron diffraction study of (Fe0.50Mn0.50)3(PO4)2 have been used to determine the cation distribution for the various compositions. Mn2+ preferentially enters the distorted octahedra and Fe2+ the five-coordinated sites. The site populations obtained are in agreement with general cation preference trends.