The humic substances in Bayer liquor from a bauxite alumina refinery operating at 145-150 degrees C have been separated by dialysis into seven fractions of different molecular weight and characterized by acidity, nuclear magnetic resonance (NMR) spectroscopy, infrared spectroscopy, pyrolysis gas chromatography mass spectrometry (py-GC/MS), and thermogravimetric techniques including modulated differential scanning calorimetry. The higher molecular weight fractions contained more aromatic carbon than the lower molecular weight fractions (e.g., >300 000 Da, 56.4%; < 1200 Da, 33.6%). Acidity, NMR, and infrared spectroscopy indicate that carboxylic groups are more prevalent in lower molecular weight fractions (e.g., <1200 Da, 20.1%) than higher molecular weight fractions (e.g., >300 000 Da, 9.8%). Some fractions have particularly interesting structures and properties. For example, lignin-derived humic substances seem to concentrate in the 6000-12 000 Da fraction, as indicated by lignin guaiacyl residues. Involatile polycondensed material concentrates in the molecular weight fraction >300 000 Da. This fraction could be best described as soluble char with unusual properties such as the capacity to hold occluded water above 200 degrees C.