The presence of xanthen-9-one in estuarine and marine waters is shown for the first time. A method for the quantification of this oxy-PAH is described and results are given for water samples from the North Sea. The concentrations range from 0.08 to 0.68 ng∗l−1 in the sea, while 3.7 ng∗l−1 were detected in the water of the estuary of the river Elbe. This compound can possibly be used as a tracer for anthropogenic contamination attributed to the combustion of fossil fuels, especially in automobile engines.