Since 1992, the Chemical Oceanography Unit of the University of Liège has carried out on a regular basis field cruises in the Scheldt inner estuary and the river plume (outer estuary), during which were measured: pH, total alkalinity, dissolved inorganic carbon, partial pressure of CO 2 (pCO 2 ), dissolved oxygen and atmospheric flux of CO 2 . In the inner Scheldt estuary, pCO 2 values in the upper estuary can be as high as 9000 ppm that is about 25 times the value of atmospheric equilibrium (presently around 370 ppm). These high pCO 2 values induce a high CO 2 efflux and the entire Scheldt can emit up to 790 tons of carbon per day (tC day -1 ) to the atmosphere. The annually integrated CO 2 emission is estimated to 456 tC day -1 ( Frankignoulle et al 1998 Science 282: 434-436 ). Along the salinity gradient, dissolved inorganic carbon dynamics are dominated on one hand by nitrification at salinities around 5 and on the other hand by dilution. Total alkalinity is not conservative in the upper estuary (salinity 0 to 5) due to intense nitrification which produces H 3 O + and leads to a decrease of total alkalinity and a minimum of both pH and oxygen saturation level. For salinities higher than 5, total alkalinity has a conservative behaviour ( Frankignoulle et al 1996 Limnol. Oceanogr. 41: 365-369 ). A simple carbon budget shows that aerobic heterotrophic activity and nitrification produce similar amounts of CO 2 and can explain most of the CO 2 emission from the inner estuary to the atmosphere. The input of CO 2 from fresh water inputs represents only 10% of aerobic heterotrophic activity and nitrification and 10% of the estuarine emission to the atmosphere. The advective flux of CO 2 , from the river to the estuary and from the estuarine mouth to the North Sea are one order of magnitude lower than atmospheric exchange in the estuarine zone ( Abril et al 2000 Comptes Rendus de l'Académie des Sciences Paris 330: 761-768 ). In the outer Scheldt estuary, pCO 2 shows a distinct seasonal evolution related to the cycle ...