Many of the chlorinated ethenes (CEs) can aerobically only be converted by cometabolism, a process in which the organism converts the contaminant that it cannot use for growth as a result of the nonspecificity of one of its enzymes. For bioremediation systems, the methanotroph Methylosinus trichosporium OB3b seems to be the best choice. Based on both experimental data on the toxicity of the cometabolic conversion products and engineering considerations, it is concluded that a two step system in which growth is separated from CE conversion is practically the most feasible. Reactor choice is considered and modelling studies are performed. If for the main design parameters (influent biomass concentration, residence time, temperature and mixing properties of the conversion reactor) optimal values are chosen, good performance is obtainable.