Abstract: Six possible reaction pathways for the thermal degradation of 4-O-methyl-glucuronic acid as a hemicellulose model compound were proposed; the reactants, products, intermediates and trasistion states involved in these reaction pathways were structurally optimized and the related standard kinetic parameters were calculated. The results show that for the thermal degradation of the hemicellulose model compound, 4-O-methyl-glucuronic acid is first converted to catenulate intermediate through a ring-opening reaction with a intramolecular hydrogen transfer, the intermediate is then decomposed, with methanol, glycolaldehyde, 2-hydroxy-3-methoxy-butyl aldehyde acid, glyoxal, 2-hydroxy-butyl aldehyde acid and so on as the major products; the competitive degradation products are formic acid, CO₂, CO, 4-hydroxy-3-vinylmethylketone, methoxyethene and so on. During the thermal degradation of hemicellulose, CO₂ is likely formed through decarboxylation of unsaturated reactants or intermediates, whereas acetic acid is probably produced through the elimination of O-acetyl.