An attempt was made to interpret the experimentally observed gas release pathways in lignin thermolysis by the frontier molecular orbital (FMO) theory. A set of 20 monoaromatic molecules that mimic the moieties in Freudenberg and Harkin's model of spruce lignin were thermolyzed at 250-550 degrees C and fractional conversions from 0.0005 to near unity. Guaiacol substrate, a model of the prevalent coniferyl alcohol monomer residues in lignin, thermolyzed by two parallel paths, namely, (R1) demethanation to CH4 and catechol and (R2) decarbonylation to CO and phenol, with their relative rates in the ratio of Thermolyses of three substituted guaiacols, namely, 2,6-dimethoxyphenol, isoeugenol, and vanillin, also resulted in demethanation and decarbonylation, analogous to RI and R2 for guaiacol, but vanillin decarbonylation, to guaiacol and CO, was more akin to and roughly 1000 times faster than the single decarbonylation path (R3) of benzaldehyde to benzene and CO. Pathway RI was mechanistically interpreted as a thermally allowed pericyclic group transfer elimination of CH4. A FMO diagram for R1 suggested a dominant electronic interaction between highest occupied molecular orbital (HOMO) (methane) <-> lowest unoccupied molecular orbital (LUMO) (o-benzoquinone), with an HOMO-LUMO "gap" of similar to 11 eV. The demethanation kinetics of 2,6-dimethoxyphenol, isoeugenol, and vanillin were essentially the same as observed for guaiacol, implying that their substituents did not appreciably alter the energies of their LUMOs relative to the parent.