The earliest pottery in Schleswig-Holstein, Northern Germany, was produced by the Final Mesolithic Ertebølle culture. Radiocarbon dating of food crusts on Ertebølle pottery indicated that ceramics from inland sites were substantially older than those from the coast. Therefore, a freshwater reservoir effect was suspected. For assessing the risk of a reservoir effect in the food crust, the presence of aquatic products such as fish has to be identified. It was tested whether stable carbon and nitrogen isotopes can be used for this purpose. In the study presented here I measured the order of magnitude and the degree of variability of the freshwater reservoir effect in Schleswig-Holstein. Experiments with copies of Ertebølle vessels yielded reference material for radiocarbon dating and stable isotope analysis. Food crusts, terrestrial samples and fishbones from two Ertebølle inland sites were analysed as well. It could be shown that the inland pottery most probably has the same age as the coastal pottery. Furthermore, I will present some methodological considerations about pottery dating and a pilot study where food crusts as well as total lipid extracts were dated.