An evaluation of mass transfer phenomena in horizontal-how anaerobic immobilized biomass (HAIB) reactors is presented. The data refers to two similar HAIB reactors filled with polyurethane foam matrices for biomass immobilization: a pilot unit and a bench-scale reactor. The pilot unit of 237 1 was used for the treatment of pre-screened domestic sewage while the bench-scale reactor of 21 was fed with glucose-based synthetic substrate. Mass balances of substrate for single particles were applied to derive expressions correlating the substrate concentration in the bulk liquid to its concentration in any position of the bioparticle. These permitted the evaluation of the substrate concentration on the solid-liquid interface and the substrate concentration profiles inside the bioparticle. The data obtained were used to perform an analysis of the mass transfer phenomena involved in the organic matter conversion process. The results indicated that the solid-phase mass transfer was the limiting step of the overall substrate conversion rate in the pilot-scale HAIB reactor treating pre-screened sewage. The liquid-phase mass transfer in the bench scale reactor treating synthetic substrate mainly affected the overall conversion rate. (C) 2000 Elsevier Science Ltd. All rights reserved.