Two trace gases, toluene and chlorobenzene, were added separately to a methane-treating biofilter using an inorganic filter material. At two fixed inlet loads of methane, 16 gC/(m(3) h) and 66 gC/(m(3) h), the influence of the concentration of toluene and chlorobenzene on methane removal was investigated. Constant elimination capacities of methane were achieved at the lowest inlet load of methane whatever the inlet load of toluene or chlorobenzene were, with 7.6 +/- 0.4 gC/(m(3) h) for toluene-biofilter and 5.9 +/- 0.4 gC/(m(3) h) for chlorobenzene-biofilter. Elimination capacities of methane were more affected by the trace gases for the highest inlet load of methane with a decrease from 13.6 +/- 1.0 gC/(m(3) h) to 1.4 +/- 0.5 gC/(m(3) h) for toluene-treating biofilter and from 11.1 +/- 0.5 gC/(m(3) h) to 4.0 +/- 0.6 gC/(m(3) h) for chlorobenzene-treating biofilter. A maximum elimination capacity of 49 gC/(m(3) h) for toluene was achieved for the lowest inlet load of methane while only 36 gC/(m(3) h) for toluene was reached for the highest inlet load of methane. A maximum elimination capacity of 2 gC/(m(3) h) of chlorobenzene was obtained for the two inlet loads of methane. This study shows that methane biofiltration may be disrupted by the presence of a trace gas in a long term operation depending on the inlet load of methane. (C) 2012 Published by Elsevier B.V.