The Moving Bed Biofilm Reactor (MBBR), where the biomass is attached to small plastic carrier elements that move freely along with the water in the reactors, has been tested for biological pretreatment of a complex chemical industry wastewater. The MBBRs were tested at organic loads up to 53 g BOD5/m2d and always removed the easily biodegradable BOD fraction, ranging from about 60 to 80%. At organic loads from 10 to 20 g BOD5/m2d slowly biodegradable organic matter was also metabolized, sometimes removing more than 95% BOD5. After polishing in an activated sludge unit, the final effluent had an average concentration of only 3.4 mg filtered BOD5/l. With a very high specific biofilm surface area, MBBRs followed by activated sludge offer a compact process combination for complete biological treatment of chemical industry wastewaters.