Abstract New phospho-silicide CoSi 3 P 3 has been synthesized and its crystal structure determined by single crystal X-ray diffraction ( R = 0.035, R W = 0.042). CoSi 3 P 3 ( Z = 4) crystallizes with the pseudo-orthorhombic symmetry and meriedric twinning in the space group P 2 1 . Lattice parameters are a = 5.899(1) A, b = 5.703(1) A, c = 12.736(1) A, β = 90°. The crystal structure can be described by zigzag chains of octahedra occupied by Co atoms alternating with zigzag chains of tetrahedra occupied by P or Si atoms. Single crystals of this phase have been investigated at room temperature by polarized Raman spectroscopy in the 100–600 cm −1 range. The results confirm the monoclinic symmetry of CoSi 3 P 3 . Magnetic susceptibility and electronic conductivity have been measured between 300 and 10 K. CoSi 3 P 3 is diamagnetic at room temperature and paramagnetic below 65 K. CoSi 3 P 3 is semiconducting at room temperature with an energy gap of about 0.12 eV.