A dynamic model was developed to describe the behaviour of primary and secondary substrates in a biofilm reactor. The model incorporates structured kinetics to describe the generation and consumption of reducing power in the catabolic and respiratory subsystems, respectively. Secondary substrate transformation through oxygenolytic or reductive mechanisms can be modelled under either single or dual limitation of the electron donor and electron acceptor substrates. An example simulation of a theoretical biofilm system was performed. (C) 1998 John Wiley & Sons, Inc.