In this work, a detailed kinetic modeling and simulation of thermal decomposition of guaiacol as a lignin model compound was carried out using accurate computational tools. Particularly, main reaction pathways of the guaiacol decomposition were explored by using an accurate composite electronic structure method, namely CBS-QB3. Thermodynamic properties of all species involved and rate constants for all unimolecular decomposition reactions were derived using canonical statistical mechanics. The calculated numbers are in good agreement with scattering data available in the literature; thus they can be confidently used in a wide range of conditions. The effects of temperature and pressure on the thermal conversion were investigated within the framework of Quantum Rice-Ramsperger-Kassel (QRRK) theory with modified strong collision (MSC) model. The resulted kinetic sub-mechanism, consisting of thermodynamic and kinetic data derived from the considered reaction pathways, was coupled to an extended kinetic mechanism to obtain time-resolved species profiles for the pyrolysis at several practical conditions in an attempt to shed more light on the nature of the complicated process. The reaction pathway analysis showed that guaiacol primarily decomposes to hydroxy-cyclopentadienyl radical (C5H4OH) and carbon monoxide (CO) via the initial homolytic cleavage of the O-CH3 bond to produce methyl and hydroxyphenoxy radicals. Such an insight into the mechanism helps to understand its chemistry better and thus effectively construct a complete detailed kinetic model for the thermal decomposition of biomass lignin.