The influence of O₂ concentration on mineralisation processes was examined by sediment incubation experiments under controlled laboratory conditions over a sequence of oxic, anoxic and then reoxidising conditions. Sediments were studied from five locations representing marine, lagoonal and brackish environments in the Gulf of Trieste (northern Adriatic Sea). A complete depletion of dissolved O₂ and nitrate were observed after ~6 –15 days in marine and lagoonal sediments, and after 5 days in brackish sediments. During the reoxygenation phase, nitrification occurred at some sites, as evidenced by increases in NO₃^– concentrations, but not at other sites, indicating an inhibited recovery response to intermittent oxic conditions. NH₄⁺ and PO₄^3– regeneration during mineralisation was much more extensive in sediments overlain by oxygen-depleted waters, driving additional eutrophication. During reoxygenation, an influx of phosphate was observed in all three sedimentary environments, removing soluble phosphorus through coagulation and finally precipitation. Two degraded organic matter (OM) types could be distinguished: OM degraded under oxic conditions in marine and brackish sediments v. OM rapidly degraded under anoxic conditions in lagoon sediments.