Extracellular enzymes play a central role in the breakdown of organic pollutants. In wetlands constructed to treat wastewaters, supplementing the naturally occurring soil enzymes may result in faster pollutant removal, or breakdown of novel pollutants, but only if the added enzymes could retain their catalytic activity. In this study, the persistence of exogenous enzyme supplements was investigated. Adding cellobiohydrolase and beta-glucosidase to sterilised soil increased enzyme activity (range 375-4210%); although the increased activity began to decline after just 10-15 days. Thus, without an active microbial population, enhanced enzyme activity is unlikely to be long lived. However, with the naturally occurring soil microbes present to maintain the improved biodegradative capacity, cellobiohydrolase and beta-glucosidase additions created significant increases (range 173-530%) in activity and these persisted for more than 6 weeks. These findings therefore support the proposal that enzyme additions can enhance enzymic biodegradation processes, and suggest that this may be achieved primarily through a 'pump-priming' mechanism.