BACKGROUND: Underground car parks are confined spaces where traffic generates many pollutants such as toluene, ethylbenzene and xylene (TEX) and nitrogen oxides (NOx). The objective of this work was to evaluate the removal efficiency of low concentrations of TEX using planted and non-planted biofilters. Two pilot-scale units of biofiltration were set up. The gaseous mixture was generated to obtain a total TEX concentration of 600 mu g m-3. Elimination performance of both biofilters was monitored for 96 days. Microbial behavior and the potential capacity to degrade the NOx, were investigated. RESULTS: The removal efficiency (RE) was 30% in the non-planted and 15% in the planted biofilter at day 3. The RE decreased more rapidly in planted biofilter than in non-planted until nutrient addition at day 77. Consequently, the RE increased to 70% in both reactors. The potential for carbon mineralization was higher than those found in soils. The denitrification activity indicated good conditions for synthesis and maintenance of the denitrifying enzyme pool in the mound of soil. This suggested that NOx could also be treated in such a system. CONCLUSION: The efficiency of the planted biofilters in removing TEX was evaluated and found to have good potential for improving air quality. Copyright (c) 2012 Society of Chemical Industry