Phthalic anhydride is mostly produced by selective oxidation of o-xylene fed as a gas. Because of the high exothermicity of the reaction, multitubular reactors cooled by molten salts are the standard technology. However, hot spots with associated reduced selectivity and catalyst deactivation are detriments in practical operation. A more nearly optimal temperature program can be realized by changing the design by addition of an adiabatic reactor for finishing the conversion. A consortium of Elektrochemische Industrie GmbH, Wacker Chemie GmbH, and Lurgi Öl Gas Chemie GmbH developed this technology, based on a monolithic finishing reactor (143). A monolith is attractive because of the low-pressure drop and the ease in integrating it in the heat-exchanger train that is commonly used in phthalic anhydride production plants. The monolithic postreactor was installed in India at a plant with a capacity of 60,000 tons of phthalic anhydride per annum and was run successfully. The monolithic finishing reactor is well suited to retrofitting in existing plants. New phthalic anhydride plants of Wacker/Lurgi are equipped with this postreactor system as standard parts of the design.