Properties of the clayey alluvial soils of the Niksar basin located at the right bank of the Kelkit River were investigated. The study consisted of both field and laboratory investigation programs. During these investigations, soil profiles were constructed and distribution of clayey soils were identified. Physical, index, mineralogical chemical properties of the soils were determined and their effects on the geochemical characteristics were investigated. Predominant minerals in the soils were Ca-smectite, kaolinite and illite type of clay minerals and quartz, calcite, feldspar. Differences in the cation exchange capacity values were due to the type and quantity of the clay mineral present in the soils. Possible heave and swelling pressure of these expansive soils may cause major damage to the structures. The soils have moderate to high swelling potential. Based on their consolidation characteristics, the soils have high strength due to their over-consolidated nature and thus significant consolidation problems are not anticipated.