The 1070-K phase relations in the M–Mn–P (M = Cr, Mo, W; ≤67 at. % P) and Re–Mn–P (≤50 at. % P) systems were studied by x-ray diffraction. The existence of the known phosphides Mo4Mn6P5 and (Mn,W)12P7 was confirmed, and a new phase with the general formula (Mn,Mo)12P7 and stoichiometry range Mo4.6–1.8Mn7.4–10.2P7 was identified. Its structure was determined by powder diffraction profile analysis (R = 0.074): Cr12P7 structure type, sp. gr. P6, a = 0.94929(8) nm and c = 0.33565(3) nm for Mn7.4Mo4.6P7 . The Mo and Mn atoms in the structure of this phase were shown to occupy all positions at random. In addition to the known solid solutions (Cr,Mn)3P (Ni3P structure type), (Cr,Mn)12P7 (Cr12P7), (Cr,Mn)P, and (Mn,Mo)P (MnP), we revealed new solid-solution series: (Cr,Mn)2P (Cr2P), (Mn,Cr)2P (Fe2P), (Mn,Mo)3P (Ti3P), (Mo,Mn)3P (α-V3S), (Mo,Mn)P (WC), (W,Mn)P (MnP), (Mo,Mn)P2 , and (W,Mn)P2 (MoP2). The Re–Mn–P system was found to contain neither ternary phosphides nor solid solutions.