The solid solution, sodium [iron(III)/manganese(II)] tris-(orthophosphate), Na₃.₄Mn₀.₄Fe₁.₆(PO₄)₃, was obtained using a flux method. Its crystal structure is related to that of NASICON-type compounds. The [(Mn/Fe)₂(PO₄)₃] framework is built up from an (Mn/Fe)O₆octa-hedron (site symmetry 3.), with a mixed Mn/Fe occupancy, and a PO₄ tetra-hedron (site symmetry .2). The Na⁺ cations are distributed over two partially occupied sites in the cavities of the framework. One Na⁺ cation (site symmetry -3.) is surrounded by six O atoms, whereas the other Na⁺ cation (site symmetry .2) is surrounded by eight O atoms.