The supposition that the Slack, a river in northwestern France discharging directly into the sea, has a significantly higher chlorinity in its bottom sediments than in the overflowing river water, is confirmed. A device to take samples of sediment from the river bottom has been constructed and is described in this publication. To obtain the interstitial water, the sample is simply centrifuged. A peculiar phenomenon, which could not be explained, is that after a regular decrease of the interstitial chlorinities, a considerable increase (to > 1.5 ‰) takes place at a distance of some 2½ kilometres from the mouth. The interstitial chlorinity in the estuarine part of the river proved to be almost constant, regardless of the tidal cycle.