A method of determining the actual and potential denitrification rates in activated sludge samples by measuring accumulation of nitrous oxide after inhibition by acetylene was adapted and applied.Rates obtained with the acetylene inhibition technique were compared with those calculated through mass balances of total nitrogen from process data as well as those measured as nitrate utilisation.The acetylene inhibition technique was a reliable method for determination of denitrifying activity.The influence of acetate, ethanol, methanol and nitrate additions was examined.Acetate gave the best response upon denitrification followed by ethanol and methanol.The bacteria in the activated sludge could not denitrify at full capacity due to limitations in the supply of electron donors.The actual rates measured with the acetylene inhibition technique corresponded to the rates obtained from process data.The potential rates, determined with additions of acetate and nitrate in excess (240 mg l−1 and 28 mg N l−1), were 3–7 times higher than the actual rates and the rates calculated from process data.There is, thus, a potential for a more rapid denitrification rate.The potential denitrification rate measured with the acetylene inhibition technique was equivalent to the nitrate utilisation rate.