The partial substitution of potassium atoms by strontium atoms in initial matrix K3Fe2(PO4)(3) with formation of complex phosphates K3-2xSrxFe2(PO4)(3) (x = 0.75, 1.0, and 1.25) was determined using a solid-state reaction method. According to powder X-ray diffraction (XRD) data, the prepared phosphates with x = 1.0 and 1.25 crystallize in a cubic system (space group P2(1) 3) and belong to langbeinite family compounds, while for the sample with x = 0.75 a mixture of langbeinite-type phase and a minor amount of K3Fe2(PO4)(3) was obtained. The decreasing trend of lattice parameter a with increase of Sr amount in the phosphate confirmed the incorporation of strontium in the langbeinite-related structure. The thermal analysis results indicate that the obtained langbeinite-related compounds are stable upon heating and their melting points are higher than 1000 degrees C. It should be noted that the determined iron-containing phosphates can be obtained at temperature lower than 900 degrees C, which creates a perspective for using them as matrixes for immobilization of radioactive strontium.