Thermal reactions of six monomeric aromatics (typical primary pyrolytic products of beta-O-4 lignin dimers) were studied in a closed ampoule reactor (N-2/400-600 degrees C/120 s) to obtain the information on secondary pyrolysis pathway of products. The result showed that the secondary pyrolysis of monomeric aromatics mainly included two pathways: polymerization and side-chain-conversion, and the polymerization were more competitive under all the test conditions. The amount of -OCH3 group was an essential factor on the reactivity of monomeric aromatics, and the introduction of -OCH3 group enhanced the conversion of monomeric aromatics. In the presence of -CHO group on the p-position of aryl-OH, the removal of -CHO group was major side-chain-conversion reaction during the pyrolysis process. Adding the amount of -OCH3 group was beneficial for promotion the breakage of aryl-CHO bond; instead, when the p-position of phenolic hydroxyl group was -COCH3 group, the cleavage of aryl-COCH3 bond was inhibited, where radical induced rearrangement of aryl-OCH3 group into aryl-CH3 group firstly happened and as the major side-chain-conversion reaction. Besides, new reactant intermediates, which were directly formed via side-chain-conversion of initial samples, could further decompose to other aromatic monomers, and influenced the products distribution and selectivity via a series of reactions.