This study demonstrates that at low to medium isobutyraldehyde loading rates (191 gm(-3) d(-1)-933 gm(-3) d(-1)), 100% removal efficiencies can be obtained in a compost biofilter. However, increasing the loading rate to 1500-1900 gm(-3) d(-1) caused a drop in degradation efficiency, a pH decrease and production of isobutyl alcohol and isobutyric acid. Additional batch and continuous experiments were performed to study the effect of pH and compost moisture content on the biofiltration of isobutyraldehyde, isobutyl alcohol and isobutyric acid. It was shown that the degradation rate of the three compounds decreased in the order isobutyraldehyde > isobutyl alcohol >> isobutyric acid, with no significant degradation for isobutyric acid. The isobutyl alcohol degradation rate was negatively influenced by the presence of isobutyraldehyde, while isobutyraldehyde degradation was not affected by the presence of either of the two compounds. A pH of 5.2 apparently inhibited the isobutyl alcohol degradation and lowered the isobutyraldehyde degradation rate, although adaptation of the microorganisms to low pH seemed to occur in the biofilters. Moisture content had a smaller effect on the degradation rates, although continuous experiments showed that a very high water content (55% compared to 40%) negatively affected isobutyraldehyde elimination increasingly during the course of the experiment. As a conclusion, it appears that at high loads of isobutyraldehyde, isobutyric acid is accumulated in the biofilter, resulting in a drop of pH. Consequently, isobutyraldehyde removal efficiency decreases and both isobutyl alcohol and isobutyric acid are measured in the effluent. It is suggested that next to moisture control, a pH buffer is necessary to remove high loads of isobutyraldehyde and to avoid persistence of intermediates in the effluent.