The performance at low water availability of styrene-degrading biofilters with the fungus Exophiala jeanselmei growing on perlite, the inert support, was investigated. E. jeanselmei degrades styrene at a water activity of 0.91-1. In biofilters, the styrene elimination capacity at a water activity of 0.91 is 5% of the maximal elimination capacity of 79 g m(-3) h(-1) (water activity 1). Application of dry air results in a rapid loss of styrene degradation activity, even at 40%-60% (w/w) water in the filter bed and at a water activity of 1. Humidification of the gas and an additional supply of water to the filter bed are necessary to maintain a high and stable styrene elimination capacity.