Partial denitrification and the initial step of nitrification are the main biological processes which produce nitrous oxide. In order to determine the contribution that these processes have in nitrous oxide production, the efficiency of different inhibitors on nitrifying activity has been tested, and the effect on denitrifying activity has been investigated, using culture strains and natural marine samples. A good nitrification inhibitor should not affect denitrification. A low partial pressure of C2H2 provided the best conditions, inhibiting 75%, Nitrosococcus oceanus (culture sample) and 100% (natural sample) of the nitrifying activity and having only a small inhibitory effect (12%) on denitrifying activity. These conditions have been applied on samples from the dilution plume of the Rhĵne River, an area characterized as a source of nitrous oxide. Using these inhibitors, it has been shown that in this area, incomplete denitrification is the main process producing nitrous oxide in the surface layers at the mouth of the river and in the bottom nepheloid layer, whereas in the marine surface layer the dominant process is nitrification.