The mineralogical composition of clay fractions (< 2μ) separated from some 50 cultivated soils from different parts of Sweden has been determined by means of X-ray, DTA, cation exchange capacity, and nonexchangeable K. Illite was found to be the main mineral. Other minerals occurring in the clays, though not always present, are quartz, kaolinite, chlorite, vermiculite, montmorillonite, feldspar, and amphibole. Certain colloidal properties of the clays were determined, such as pH of the H-clay, floe volume (sediment volume), swelling in and absorption of water, hygroscopicity, cation exchange capacity, and particle size distribution. These properties were found to be rather well correlated to the mineral composition.