A process for preparing carboxylic acids and/or carboxylic anhydrides by gas phase oxidation of aromatic hydrocarbons, in which a gas stream comprising at least one aromatic hydrocarbon and molecular oxygen is passed continuously over a catalyst thermostatted by a heat carrier medium, which comprises keeping the temperature of the heat carrier medium constant during the startup of the reactor for at least 24 hours, during which neither the loading of the gas stream with hydrocarbons nor the gas stream volume is increased by more than 3%.