Temperature-programmed decomposition, thermal analysis, thermogravimetry and high-temperature X-ray analysis have been used to investigate a commercial catalyst for the selective oxidation of benzene into maleic anhydride. It was shown that phase transformations took place in the vanadia-molybdena alkali-promoted catalyst. The transformations were caused either by high-temperature treatment or by catalytic work in an industrial plant. As a result, theβ-bronze phase was formed. In the high-temperature region a reversible transformation of the bronze phase was observed. This was interpreted as theβ⇋β′ phase transformation.