K-Prima, Witbank, Warkworth, Goonyella, Luscar, and Pittston-MV coal samples were heated to temperatures between 200 and 550 degrees C at a rate of 3 degrees C/min in an autoclave under nitrogen and then cooled rapidly to room temperature. Heat-treated coals were extracted with a carbon disulfide-N-methyl-2-pyrrolidinone mixed solvent at room temperature; the extracts were fractionated further with pyridine and chloroform. Raw coals contained a significant fraction of solvent-soluble constituents (20-45 wt %). For high-caking coals, the amount of heavy solvent-soluble constituent, i.e., the pyridine-insoluble and the mixed solvent-soluble fraction, greatly increased at the fluidity stage. There is a good linear relation between the maximum extraction yield and the Gieseler fluidity of the coals. This result suggests that heavy constituents are an important factor for the occurrence of fluidity in coals.