ABSTRACTSemi‐quantitative clay. mineral analysis was carried out on the clay and silt fractions of approximately three hundred Black Sea core samples. Relative abundance of montmorillonite, illite, kaolinite and chlorite was determined.Illite is the most frequent clay mineral in the Black Sea surface sediments. Highest values are obtained in the northern and central Black Sea. Approaching the Anatolian coast, the illite portion gradually decreases at the expense of montmorillonite. Chlorite and kaolinite occur generally only in small quantities. The lateral changes in the composition of the clay minerals can easily be traced back to the petrology of a northern (rich in illite) and a southern (rich in montmorillonite) distributive area.In almost all cores a periodical fluctuation of the montmorillonite/illite ratio with depth could be observed which may be related to the changing influence of the two distributive provinces during the Holocene and Late Pleistocene (Würm). Higher montmorillonite contents indicate arctic and subarctic climate periods in the northern distributive area during which the illite supply was diminished to a large exte