From September 1990 through December 1991 nitrous oxide flux measurements were made at 9 intertidal mud flat sites in the Scheldt Estuary. Nitrous oxide release rates were highly variable both between sites and over time at any one site. Annual nitrous oxide fluxes vary from about 10 mmol N m-2 at the tidal fresh-water end-member site to almost zero at the most saline stations. Along the estuarine gradient, annual nitrous oxide fluxes are significantly correlated with sedimentary organic carbon and nitrogen concentrations, ammonium fluxes and annual nitrogen turn-over rates, that are estimated using mass-balance considerations. Nitrous oxide fluxes seem to respond linearly to an increasing nitrogen load, with one out of each 17 000 atoms nitrogen entering estuaries being emitted as nitrous oxide. © 1995 Kluwer Academic Publishers.