Ammonia and hydrogen sulfide emissions from hog farms is common and it can be a damaging impact environmentally and economically.  To reduce the quantity of gaseous emissions, biofilters have been ustilized.  It is known that biofilters are able to reduce emissions but little work has been done to evaluate the process which cause the reduction.  Possible mechanisms include chemical, biological or a combination of both.  To determine mechanisms in the filters treatment were established which include: a blank, an untreated biofilter, a biofilter treated with cholroform to reduce microbial activity, a mixed resin, a resin that has been sprayed with liquid peat, and a biofilter treated with the liquid peat.  Concentrations of ammonia and hydrogen sulfide were measured to determine the effect of the biofilters for reducing gaseous emissions.