Transient behavior of a biofilter was observed to eliminate hydrogen sulfide and ammonia simultaneously from waste air at its four sampling ports. The biofilter was packed with a eqivolume mixture of granular activated carbon(GAC) and compost as packing media, on which Thiobacillus sp. IW and return-sludge from wastewater treatment plant were inoculated and were fixed. The biofilter was run under various operating conditions. After the inlet load of hydrogen sulfide surpassed ca. 100 S-g/m3/h the removal effciency of hydrogen sulfide began to decrease and was finally continued at 93%, which was very similar to that of the case when waste-air contained only hydrogen sulfide. However the removal efficiency of ammonia was much less than that of the case when waste-air contained only ammonia so that it decreased to 53% as the inlet load of ammonia increased to 44 N-g/m3/h.