The annual cycle of oxygen in 1986 was studied in the western Wadden Sea by full tide measurements at representative locations in the tidal channels. In winter a situation close to saturation was encountered, whereas in the period May to October substantial deviations from saturation occurred. In late May in the outer Marsdiep basin oxygen supersaturation up to 148% was found, which was caused by a high rate of phytoplankton primary production. During most of the summer and autumn undersaturation occurred, but only to a minor extent. There was a general gradient of decreasing oxygen towards the basin interior during this period. Near the IJsselmeer sluices which discharge freshwater into the area, some strikingly deviant tidal cycles of low oxygen were observed. These were probably caused by the decomposition of large amounts of dying freshwater phytoplankton. On an annual basis the Marsdiep basin was calculated to be a pronounced sink for oxygen, the deficit amounting to 8·1 μmol 1 1. Budget and transport calculations resulted in two independent estimates of 'excess mineralization' in the Marsdiep basin of 132-138 × 106 kg O2 year 1. The supply of oxygen from the atmosphere amounted to more than half this figure, the remainder being supplemented through the tidal inlet. © 1991.