X-ray powder diffraction measurements were made at room temperature on sixteen polycrystalline samples of I-2-Fe-IV-VI4 (I: Cu, Ag; IV: Si, Ge, Sn, Ph; VI: Se, Te) magnetic semiconductor compounds. The diffraction patterns were analyzed to determine values of lattice parameter for each compound. The results showed that Cu2FeSiSe4, Cu2FeGeSe4 and Cu2FeSnSe4 have the tetragonal stannite structure (I (4) over bar 2m), while the rest of the materials have an orthorhombic superstructure of wurtzite which is known as wurzt-stannite (Pmn2(1)). It was found that, when the values of the effective parameter a(e) = (V/N)(1/3) are plotted against the molecular weight W of the materials, the tetragonal and orthorhombic compounds lie on different straight lines. In addition, differential thermal analysis (DTA) measurements were made, and the peaks on the DTA cooling curves were used to determine values corresponding to the melting temperature for the compounds.