The diagenesis in fine-grained sediments from a 300 to 400-years-old Dunkirk deposit, exposed on the intertidal flat, was studied at a site in the Eastern Scheldt. A new in situ pore water sampling technique that allowed repeated sampling at exactly the same place was used to monitor the seasonal fluctuations in interstitial water composition. Concentrations of organic carbon (1.5 to 2%), nitrogen (C/N = 19), phosphorus (500 g?g−1) and manganese (250 g?g−1) in the subrecent anoxic sediments were low, probably because they had already been depleted during earlier stages of diagenesis. Rates of organic carbon mineralization by sulphate reduction (0.1 Mole?m−2?y−1) and rates of nutrient regeneration were 1 to 2 orders of magnitude lower than in recent fine-grained sediments elsewhere in the Eastern Scheldt. Pore water NH4 and PO4 concentrations were controlled by mineralization, uptake by Zostera noltii and sediment-seawater exchange. During the summer the uptake exceeded the mineralization rate at 0 to 5 cm. Mineralization and diffusional processes dominated the changes in the NH4 and PO4 profiles in the other seasons. Dissolved manganese and iron concentrations showed a typical subsurface maximum at 0 to 3 cm, and low (<5 Mole) concentrations below this depth. Dissolved iron concentrations were probably controlled by the solubility of iron sulphides, and manganese probably by the solubility of Mn, Ca-carbonate.