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In this study, the formation mechanism of light aromatics including benzene, toluene, ethylbenzene, xylene, and naphthalene (BTEXN) and the coke growth during catalytic reforming of thermally derived volatiles of cellulose, glucose, and lignin over HZSM-5 under a H 2 atmosphere were investigated rigorously. The shape selectivity of HZSM-5 coupled with H 2 as the promoted agent was found to be highly efficient in trapping volatiles to promote the production of BTEXN. The reactivity of the three biopolymers from biomass structures was compared over HZSM-5 to reveal the hydrocarbon pool and the phenolic pool mechanisms, which are the key mechanistic steps for the BTEXN products and the undesired coke. The results of the present work indicated that there are competing adsorption between the reactions of the aromatics formation and the reactions responsible for the catalytic coke. The formation of both the "soft" and the "hard" coke species strongly depends on biomass structures. The main coke species on spent HZSM-5 are polyaromatics formed via ring growth and the heavy aliphatic species with longer chain (C 10 −C 30 ) formed via chain growth of the C 6 −C 10 oligomers. By analyzing the coke location and the coke nature, we proposed the coke growth mechanism in terms of the aliphatic-based cycle and the aromatic-based cycle.