The results of the qualitative and semiquantitative determination of clay minerals in the Keuper clastics and carbonate rocks (sandstones, shales and dolostones) are presented and their depositional environments deduced. Well crystallized illite, and chlorite are the most predominant clay minerals in the Keuper rocks, followed in amount by very well crystallized kaolinite, and palygorskite. It is suggested that kaolinite was formed in a fluvial environment, illite and chlorite were formed under marine to shallow-marine conditions, or perhaps were derived from older sedimentary rocks. Palygorskite was formed under arid evaporatic conditions. The clay minerals were most probably formed either from the weathered products of the parent rocks (i.e. igneous and metamorphic rocks of the crystalline cores of the Western Carpathians, and/or from the foreland of the Bohemian Massif) or as a result of change in conditions of depodition.