Biofilters are a relatively inexpensive management method for biodegradable gas-phase contaminants, capable of meeting stringent air quality requirements. However, the nominally plug flow configuration of conventional unidirectional flow (UF) biofilters results in the microbial community having higher population densities and greater activity near the inlet. When transient loadings occur in conventional biofilters either the mass transfer capacity or the reaction capacity of the initial sections of the bed may be exceeded and contaminants move into the downstream sections where the microbial populations and reaction capacities are low and contaminant breakthrough may occur. Flowdirectional-switching (FDS) operation incorporates the advantages believed to accrue from feast/famine operation of microbial processes and improved mass transfer characteristics of uniformly distributed biofilms. In this study, step function changes in toluene concentration were applied to UF and FDS laboratory reactors operated in parallel. Contaminant concentration was monitored at several points along the packed beds. FDS operation produced a more uniform (dispersed) distribution of biomass and microbial reaction capacity along the length of the packed bed without diminishing activity and removal capacity in the inlet section. Maximum mass removal rates in the FDS biofilter were approximately twice that of the conventional UF biofilter. In addition, FDS operation significantly improved biofilter response during an extended period of operation with intermittent feed and following a period of non-operation. (c) 2005 Elsevier B.V. All rights reserved.