Based on the investigation of phases present and luminescence of the system Sr 8 [Si 4 x O 4 + 8 x ]Cl 8 :Eu 2 + , the correct formula for this material was determined as Sr 8 [Si 4 O 1 2 ]Cl 8 :Eu 2 + rather than Sr 4 Si 3 O 8 Cl 4 :Eu 2 + , which had been assumed previously. The structure of this material was refined by the Rietveld method using powder X-ray diffraction data, which has a tetragonal unit cell with space group I4/m, Z = 2, a = 1.11814(1)nm, c = 0.95186(1) nm. In this structure, Sr has one site; therefore, when Eu 2 + is doped in, the luminescence gives only one broad emission peak.