Alkalinity distributions were measured seasonally in the western Wadden Sea and in the Southern Bight of the North Sea. Although the distributions varied considerably both spatially and temporally the relationships with salinity in the areas were linear. The large scattering and the sometimes simultaneous discrepancies between extrapolated and measured freshwater alkalinity are predominantly ascribed to short-term variations in freshwater alkalinity. Only in the Wadden Sea in May was apparent non-conservative behaviour of alkalinity concluded. An analysis of the large scattering revealed more contributors, such as additional (high-alkalinity) freshwater sources and alkalinity supply from the sediments. In the North Sea distributions were mainly scattered early in the year, largely caused by strong variations in the Rhine alkalinity. Alkalinity allowed a discrimination of the plumes of the rivers Rhine and Scheldt (the alkalinity of the latter being much higher). Likewise the salinity-alkalinity plots for Wadden Sea and North Sea were sufficiently different. The actual use of alkalinity as a tracer, however, is strongly hampered by freshwater variations and several additional high-alkalinity sources. © 1990.