The role of shelf seas in global carbon cycling is poorly understood. The dissolved inorganic carbon system and air-sea exchange of carbon dioxide (CO2) are described for the Dutch coastal zone in September 1993. The inorganic carbon chemistry was affected by tidal mixing, wind speed, wind direction, freshwater input, stratification and coastal upwelling. Surface water had a variable fugacity of carbon dioxide (fCO2) between 300 and 800 μatm with short-term changes partly related to the tidal cycle. High contents of dissolved inorganic carbon (DIC) and CO2 in relatively saline water probably originated from mineralisation of accumulated organic matter in water and sediments farther out at sea and transport of water enriched in DIC into the coastal zone by upwelling. Air-sea exchange of CO2 ranged from —20 to 60 mmol m−2 day−1. These fluxes are critically discussed in the light of potential stratification. It is not possible to assess from this study whether the Dutch coastal zone is a net sink or source for atmospheric CO2.