The invention relates to a method for converting a precatalyst which comprises a inert carrier, an organic carbon source and a silver and vanadium containing multimetal oxide to a gas phase oxidation catalyst comprising the inert carrier and a catalytically active silver-vanadium oxide bronze. According to said method, the precatalyst is thermally treated at a temperature of at least 350 DEG C in a gas atmosphere containing less than 10 % by volume of oxygen. Before the treatment, the carbon source quantity in the precatalyst is adjusted to a value below the critical quantity. The carbon quantity is reduced by separation at a temperature of 80 to 200 DEG C in an oxygen-containing atmosphere and concurrent decomposition of a portion of the carbon source. The catalysts so obtained are used for partial gas phase oxidation of aromatic hydrocarbons to aldehydes, carboxylic acids and/or carboxylic acid anhydrides.