To investigate the lignin pyrolysis mechanism, two a alpha-O-4 and one completely substituted beta-O-4 lignin dimeric model compounds were studied using in situ synchrotron vacuum ultraviolet photoionization time-of-flight mass spectrometry (SVUV PIMS) at 350-500 degrees C. The collision-reduced vacuum condition, in situ characteristic, and "soft" ionization technique of this reactor system allowed for the direct detection of thermolysis radicals and high-boiling-point compounds. For the alpha-O-4 compound 4-(benzyloxy)phenol (BOP) pyrolysis, benzyl radical, p-semiquinone radical, toluene, and bibenzyl were confirmed, supplying firm evidence for the free radical dominant mechanism. Experiments of the p-methoxy substituent on the hydroxyl position of BOP show that the p-methoxy substituent can lower the C-aromatic -C alpha and ether C-aromatic bond dissociation energies. For beta-O-4 compound guaiacylglycerol-beta-guaiacyl ether (GGGE), it is inferred that the C-O homolysis mechanism was minor and the concerted reaction dominated in the experimental temperature range. Also, parallel experiments were conducted to find the temperature effect on the product distribution; it reveals that a high temperature leads to the further decomposition of the primary products and an increase of the aldehyde and ketone components.