This work is the first extensive study on the removal of carbon monoxide from polluted air in biofilters. It compares the performance of two packing materials, namely lava rock alone and a mixture of peat and lava rock. The results show that the biofilter packed with the mixture of peat and lava rock performed much better than the other one. The effect of operating conditions as, among others, the inlet concentration and the empty bed residence time (EBRT) were studied. A maximum elimination capacity of 33 g m(-3) h(-1) was obtained with the mixed packing with more than 85% removal efficiency at EBRT of 3 min or more. Somewhat lower performances were reached at shorter EBRT. The results presented here suggest that the mixture of lava rock and peat, subject to further optimization, offers potential for the biological removal of CO from polluted gas streams.