The quaternary chalcogenide compound Ag2MnSnS4 belonging to the system I2-II-IV-VI4 and synthesized by the melt and anneal technique, was characterized by Rietveld refinement of the powder X-ray diffraction data and differential thermal analysis (DTA). It was found that nS4 crystallizes in the orthorhombic space group Pmn21, with unit cell parameters a = 8:1705(5) Å, b = 6:9413(5) Å, c = 6:6532(5) Å, and V = 377:33(5) Å3, in a wurtzite-stannite structure. The DTA indicates that this compound melts at 790°C and that the phase relations which occurs in the material would be: α ➝ α + α1 ➝ α1 ➝ α1 + β ➝ β ➝ 1 + L ➝ L, were α is the orthorhombic wurtzite-stannite Pmn21 structure; α1 is a high temperature modification; and β and β1 are the zinc-blende structure and its high-temperature modification, respectively.