Results of toluene and p-xylene containing model gases biofiltration are presented. Process was carried out on columns packed with two kinds of non-modified composts. Velocity and efficiency of biofiltration was determined and plotted vs. impurities concentration and linear gas velocity. Highest, average for the top section of the column, values of the decomposition velocity were found to be ca. 80 g/m3 /h, and average for the whole column ca. 40 g/m3/h respectively. Fluctuations of these values at similar, well controlled, measurement parameters were however encounter, most probable due to varying hardly adjustable humidity of the column bed.