Abstract: The pyrolysis of 2,3,4-hydroxyl-butyraldehyde and 2,3,4-hydroxyl-butyric acid as model compounds was investigated by using B3LYP/cc-pVTZ methods to reveal the decarbonyl and decarboxyl mechanism. Three possible pathways for the pyrolysis of each model compound were designed and the standard thermodynamic and kinetic parameters of each reaction path at different temperatures were determined. The results showed that the release of CO and CO₂ during the cellulose pyrolysis is related to the decarbonyl and decarboxyl reactions, respectively; both involve a concerted process via intra-molecular hydrogen transfer. Decarboxyl reaction is endothermic while decarbonyl reaction is exothermic. The activation energy of decarbonyl reaction of 2,3,4-hydroxyl-butyraldehyde is 288.8 kJ/mol, while the activation energy of decarbonyl reaction of undersaturated olefine aldehyde after dehydration is higher than that for saturated aldehyde. The activation energy of decarboxyl reaction of 2,3,4-hydroxyl-butyric acid is 303.4 kJ/mol, while the activation energy of decarboxyl reaction of undersaturated olefine acid after dehydration is much lower, indicating that the dehydration favors the release of CO₂.