Smectite, K-smectite and illite/smectite (I/S) mixed-layer minerals are found as expandable clay minerals in the Paleogene coal seams of the Ashibetsu coal mine in the Ishikari coal field, Hokkaido, Japan. As K-smectite often exists in tuff, tuffaceous shale and coal near tuff, and only occasionally in shale, the K-smectite is simply thought to represent detrital input from tuff. Smectite often exists in the coals near intercalated tuff, roof tuff and tuffaceous shale beds, but is not found in these sediments. Although many tuffs contain both K-smectite and I/S mixed-layer clay minerals, the smectite coexists with only K-smectite in many coals and the coal in which smectite occurs never has both of the other minerals. From these relationships it may be inferred that smectite was formed in the coal mainly from the I/S mixed layer and only partly from K-smectite. The I/S mixed-layer clay found in the tuff and tuffaceous shale is never accompanied by illite but the I/S mixed-layer clay in the shale is almost always accompanied by illite. I/S mixed-layer clay without illite exists in some coals near tuff and I/S mixed-layer clay with illite is often found in the other coals. These relationships mean that the I/S mixed-layer clay in the coal is also of detrital origin, derived from the same sources as both the tuff and the shale beds. The I/S mixed-layer clays plotted using Watanabe＇s (1981, 1988) identification graph show a pattern consistent with illitization by diagenesis. The illitization appears to be related to the burial depth and/or coal rank. It is also shown that the mixed-layer clays in the shales have higher illite layer ratios than those in the tuffs, the tuffaceous shales and the coals without illite. From this evidence it appears that the composition of I/S mixed-layer clay depends mainly on processes associated with the source of the original sediment input.