N2, Ar, O2, and CH4in sedimentary pore waters of the Tama Estuary, Tokyo, were determined. The distributions of N2and Ar reflected seasonal changes in these gases in the overlying water. N2/Ar ratios were always higher than in water in equilibrium with the atmosphere, providing direct evidence for denitrification in the sediment. N2production from nitrate by bacteria in the sediment was simultaneously determined using a15N tracer technique. The distribution of the excess N2so produced was calculated using a one‐dimensional diffusion model and compared with observations. Rapid loss of N2from sediment to overlying water is suggested.