Mass transfer and reaction were studied in a lab-scale biofilm airlift suspension reactor. A new approach for the determination of liquid-solid mass transfer coefficients in biofilm systems under reacting conditions is presented. The method is based on the analysis of oxygen consumption in a biofilm airlift suspension reactor and a biological oxygen-monitoring system and allows the independent estimation of gas-liquid and liquid-solid mass transfer coefficient and biofilm reaction rate. The influence of some operating conditions (solid loading, particle size and gas flow rate) on the liquid-solid mass transfer coefficient in biofilm airlift suspension reactors was investigated and a correlation for the mass transfer to biofilm-coated particles is proposed. Liquid-solid mass transfer to biofilm particles was proven to be mainly influenced by gas velocity, whilst particle size and solid loading had minor effect. The liquid-solid mass transfer coefficient measured for biofilm-coated particles was found to be smaller (by a factor varying from 5 to 25%) than the values reported for rigid particles. (C) 1998 Elsevier Science Ltd. All rights reserved.