New phospho-silicides RhSi 3 P 3 and IrSi 3 P 3 were synthesized and their crystal structures were determined by single crystal X-ray diffraction techniques. Both compounds are isostructural at room temperatures and crystallize with the monoclinic symmetry in the space group C2. Lattice parameters are a = 6.626(1) b = 7.210(1) , c = 5.522(1) and β = 118.31(1)° for RhSi3P3 and a = 6.577(2) , b = 7.229(2) , c = 5.484(2) and β = 117.91(2)° for IrSi 3 P 3 . The crystal structure can be described by separated octahedra of Si or P atoms surrounding metallic atoms Rh or Ir. The octahedra are connected to each other by chains of type M Si/P Si/P M. The atoms of P and Si are fourfold coordinated.