The crystal structures of three ternary Mo–Pt–Si intermetallic compounds have been determined ab initio from powder X-ray diffraction data. All three structures are representative of new structure types. Both the X (MoPt2Si3, Pmc21, oP12, a=3.48438(6), b=9.1511(2), c=5.48253(8) Å) and Y (MoPt3Si4, Pnma, oP32, a=5.51210(9), b=3.49474(7), c=24.3090(4) Å) phases derive from PtSi (FeAs type) structure while the Z phase (ideal composition Mo32Pt20Si16, refined composition Mo29.9(2)Pt21.0(3)Si17.1(1), Cc, mC68, a=13.8868(3), b=8.0769(2), c=9.6110(2) Å, β=100.898(1)°) present similarities with the group of Frank-Kasper phases.