The phase relationships and structure of Sb-Sn-O and Sb-Sn-Cu-O oxidation type catalysts were determined by means of X-ray diffraction, small angle X-ray scattering and electron microscopy techniques. The addition of Cu to the Sb—Sn—O-based catalysts seems to inhibit the sintering of the solid, but it avoids the formation of weak metal-oxygen bonds, i.e. Sb(III)—O. The fundamental difference between monometallic oxides and mixed phases is the preferred development of (020) and (001) type faces, which could be relevant for the partial oxidation selectivity properties of the catalysts.