Total CO2 (TCO2) distribution patterns in the coastal North Sea in different seasons showed decreasing values towards the open sea. The higher values near the coast were caused by TCO2 outflow from the Rhine/Meuse. The highest values were observed in November, irrespective of the salinity. Mixing generally resulted in linear relationships between TCO2 and salinity, but scattering was lowest in November. Both pH (measured) and pCO2 (calculated) clearly demonstrated the influence of biological activity on the CO2 system. In November, pCO2 was very high near the coast (>450 μatm). This is attributed mainly to mixing, and only partly to mineralization. The oxygen distributions were consistent with those of the CO2-related parameters, the deviations being opposite. The data showed that, in accordance with theory, the equilibration of O2 with the atmosphere was faster than for CO2. TCO2- alkalinity diagrams were only of limited value, since normalization to constant salinity was hampered by freshwater variations of TCO2 combined with the variability of the residence time of the water masses in the coastal area. © 1991.