The performance of biofiltration to remove odours from animal rendering plant’s gaseous emissions was investigated using pilot-scale biofilters containing different media (sand, sawdust, bark, barkhoil mixture). Biofilter influent and effluent gases were characterised using a gas chromatograph-mass spectrometer (GC-MS) and a GC fitted with an odour “sniffing” port. Overall odour-removal performance of the biofilters was determined using a forced-choice dynamic-dilution olfactometer. The biofilter influent gases contained about 300 organic compounds, of which approximately 40 were odorous. The odorous compounds included alkanes, alkenes, ketones, hydrocarbons, alcohols, alkyl halides, fatty acids, amines, aromatics, aldehydes and epoxides. The biofilters reduced the concentrations of the odorous compounds, often to levels that were undetectable by the GC. Some of the odorous compounds in the biofilter effluent gas came from the biofilter medium or were the result of biological or chemical transformations within the biofilter. Biofilter odour removal efficiencies of between 75% and 99% were measured at influent odour concentrations of between 490,000 and 1,100,OOO odour units m-3, and various air loading rates (0.074-0.057 m-3 gas m3 medium mid ) and medium moisture contents. Biofilters with new media, low air loading rates, or high medium moisture contents generally gave the best odour removal. Different biofilter media gave similar odour reductions at the gas loading rates examined.