Abstract: Illite claystones (tonsteins) of Late Permian coal-bearing formation are well developed and extensively distributed in Southwestern China. Over the past decades it has been recognized that they are synsedimentary volcanic ash-falls in origin (altered tuff beds), based on the data derived from the comprehensive investigations on their stratigraphic and geographic distribution, petrograpbic types, chemical composition, accessory mineral assemblage, as wel as on their morphological characteristics. Recently, mineralogical and petrological identifications and x-ray diffraction analyses on these peculiar tonsteins further suggest that they can be classified mainly into three categories based on their clay mineral constitutions: kaolinitic, kaolinitic-illitic (transitional type), and illitie. As coal-partings in most cases, these different types of tonsteins show a distinct zonal distribution on a regional scale. Their mineral constitutions are profoundly modified by the combined effects of many factors during deuterogenic diagenesis, as indicated by the variation in volatile component percentage (Vr) of the adjacent coal seams (Vr is calculated on a combustible component basis) . Those tonsteins intercalated in coal with a Vr ranging from 10% to 12% are dominated by kaolinite. But they contain increating amount of illite at the expense of kaolinite when Vr of coal decreases from 10% to 8%. When Vr declines to less than 8% the clay minerals in these tonsteins are almost entirely illite with some sort of accompanying chloritization. It is clear that, from a regional point of view, the variation in clay mineral constitution of thses tonsteins is in response to the continual and progressive change in geological processes, during which the temperature and pressure were the dominent conditions, and the existence of a certain amount of alkali metal ions and ferrous ions in the solution is also one of the controlli ng factors.