KMg(0.09)Fe(1.91)(PO(4))(2), potassium [iron(II)/magnesium] iron(III) bis(orthophosphate), is a solid solution derived from compounds with general formula KM(II)Fe(PO(4))(2) (M(II) = Fe, Cu), in which the Mg atoms substitute Fe atoms only in the octa-hedrally surrounded sites. The framework of the structure is built up from [FeO(5)] trigonal bipyramids and [MO(6)] (M = (Fe, Mg) octa-hedra sharing corners and edges and connected by two types of bridging PO(4) tetra-hedra. The K(+) cations are nine-coordinated and are situated in channels running along [101].