Alteration of clay minerals by weathering in Tazewell, early Cary, and late Cary tills of the Northwest Allegheny Plateau was determined by X-ray technique. In each locality 10 samples closely spaced through the entire profile from fresh unweathered till to the A soil zone were studied. Each zone in the weathering profile of till is characterized by a specific clay-mineral assemblage. The well-crystallized illite in the fresh unweathered till is changed by hydration of the intermica sheets, and in the A soil zone illite-montmorillonite mixed layers are abundant. The well-organized chlorite of the fresh unweathered till is altered by hydration of the brucite sheets culminating in the development of vermiculite in the A soil zone.