A mechanistically based model of a fixed-film trickle-filter bioreactor for cometabolic degradation of TCE has been developed. This model incorporates diffusional and kinetic resistances and generates estimates of TCE stripped from the liquid by gas as well as the extent of biological degradation of TCE. The model is consistent with a limited amount of experimental data. Further experience with this modeling approach is necessary in order to determine if this is a useful procedure for correlating ioreactor performance for design and scale-up.