Abstract: Catalyst systems for preparing carboxylic acids and/or anhydrides, the catalyst system comprising a reaction zone and a layered catalyst, the reaction zone comprises a gas inlet region and a gas outlet region, the layered catalyst comprises an active composition and one or more middle layers, one or more first layers disposed on a side of the one or more middle layers toward the gas inlet region, and one or more second layers on a side of the one or more middle layers toward the gas outlet region, wherein the active composition content of one or more of the middle catalyst layers, based on total mass of the layered catalyst, is lower than the active composition content of the one or more first catalyst layers and is lower than one or more second catalyst layers; and processes for gas phase oxidation employing a layered catalyst of the present invention.