The kinetic parameters for combined carbon/nitrogen removal in a four-stage aerated submerged fixed-film (ASFF) bio-reactor of 50 l volume were evaluated using a synthetic wastewater at 20 degrees C. Influent COD concentrations were in the range of 0.057-1.8 g COD l(-1), and influent NH3-N concentrations were in the range of 0.0152-0.48 g NH3-N l(-1) with hydraulic loaning ranging from 0.028 to 0.112 m(3) m(-2) day(-1) coed nitrification was achieved even when high mass loadings were applied. The substrate (GOD or ammonia) removal in the reactor was described by a substrate-limiting phenomenon (reaction or mass transport). Substrate-removal rate followed Monod kinetics in the first stage of the reactor but was better described by first-order kinetics in the second to the fourth stages as the substrate concentration became much lower: The order of the reaction was apparently a function of the hydraulic lending, indicating the influence of mass transfer on the 'apparent' order of the reaction. Copyright (C) 1996 Published by Elsevier Science Ltd.