A process is described for converting a precatalyst which comprises an inert support, an organic carbon source and a multimetal oxide comprising silver and vanadium to a gas phase oxidation catalyst which comprises the inert support and a catalytically active silver vanadium oxide bronze, by treating the precatalyst thermally at a temperature of at least 350° C. in a gas atmosphere which comprises less than 10% by volume of oxygen, wherein, before the thermal treatment, the amount of the carbon source in the precatalyst is adjusted to a value below a critical amount The carbon content is reduced by burning-off at a temperature of from 80 to 200° C. in an oxygenous atmosphere with decomposition of a portion of the carbon source. The catalysts obtained serve for the gas phase partial oxidation of aromatic hydrocarbons to aldehydes, carboxylic acids and/or carboxylic anhydrides.