Abstract Effects of temperature, oxygen and nitrate concentrations of the overlying water, and the thickness of the sediment layer on the rate of denitrification in the sediment were investigated in two water-sediment systems, A and B. At 4°C, denitrification started after a prolonged lag period in contrast to nitrification which did not occur significantly. At 15°C, and particularly at 25°C, both processes proceeded readily. The disappearance of NO 2 − - N from the overlying water was more rapidly than that of NO 3 − - N. The denitrification rate was slightly reduced by increasing the dissolved oxygen concentration in the overlying water from 0 to approximately 2 mgl −1. A further rise of the dissolved oxygen concentration had no further decreasing effect on the denitrification rate. The denitrification rate in sediment was dependent on the nitrate concentration in the overlying water approximating first order kinetics at lower concentrations, gradually becoming independent of the nitrate concentration at higher nitrate contents (zero order kinetics). When starting with a nitrate-nitrogen concentration of 25.2 mgl −1, a sediment layer of 7 mm with A and 14 mm with B was roughly found to be involved in denitrification. Denitrification rates found in the present laboratory experiments were supposed to be considerably lower than those occurring under natural conditions as additional mechanisms for the transport of nitrate into sediments occurred in natural environments.