The use of gas chromatography/mass spectrometry (GC/MS) in the investigation of coal structures and identification for composition of small molecules (with molecular weight at or less than 500 Da) was reviewed. Macromolecules and small molecules coexist in coals. Understanding the role of small molecules is the fundamental for effective, clean and value-added utilization of coals. For coals after treatments (extraction, oxidation and liquefaction), small molecules can be extracted from the macromolecular network and analyzed using GC/MS. Variou skinds of organic species were identified using GC/MS, which gave explanation for coal structures and mechanisms for coal liquefaction and oxidation. (C) 2012 Elsevier Ltd. All rights reserved.