Stoichiometrically adjusted antimony triselenide and antimony tritelluride semiconductors were carefully prepared for the first time. Numerous measurements were carried out, involving X-ray diffraction analysis, true density, thermogravimetric analysis and the temperature-dependence of the magnetic susceptibility. The results obtained indicated that antimony triselenide and antimony tritelluride polycrystals possess mass susceptibilities of −0.361×10−6 and −0.386×10−6 C.G.S., respectively. The results are discussed on the basis of electronegativity difference, partial ionicity of the bond, and bond strength.