This study aims to demonstrate the general carbocatalytic conversion behaviors and reaction mechanisms of representative biomass-derived primary pyrolytic products towards product upgradation. Specifically, 4-Ethyl-guaiacol (4-EGC) and 5-hydroxymethylfurfural (5-HMF) were selected as lignin-and cellulose/hemicellulose-derived model compound, respectively, to elucidate the origin of products under different temperatures and biochar-reactant mixing ratios. Three reaction routes were distinguished, including side-chain elimination, ring opening and recombination, with each route exhibiting unique characteristics in terms of initiation temperature and selectivity. For 4-EGC, elimination of side-chains began at 500 degrees C, accompanied by alkyl transfer reactions to generate multi-alkyl-substituted phenols, while ring-opening started at 700 degrees C, and aromatics could be simultaneously produced. Biochar did not change the initiation temperature of each route but significantly influenced the product distribution. Comparing the biochar-reactant mixing ratio of 0 and 10:1, at 600 degrees C the selectivity towards phenols increased from 20.52 % to 54.98 %, and at 800 degrees C the selectivity towards aromatics increased from 14.28 % to 53.01 %. For 5-HMF, the elimination of side chains already began at 400 degrees C, and the ring -opening started at around 600 degrees C, but recombination products (phenols and aromatics) would not notably emerge until 700 degrees C. Biochar could both lower the initiation temperature of each reaction route and change the conversion efficiency for 5-HMF owing to the lower stability of furan ring. However, the impact of biochar on selectivity towards aromatics was less pronounced compared to 4-EGC. Even at 800 degrees C, the relative content of aromatics only reached 28.88 % with a biochar-reactant mixing ratio of 10:1 for 5-HMF. Comparative in-vestigations on biochar structure before and after co-pyrolysis reactions further revealed that, at 600 degrees C, biochar maintained its structural integrity, but at 800 degrees C, it underwent significant changes, including amorphous carbon deposition covering the surface. Raman spectra analysis supported these findings, showing that biochar becomes more amorphous after reactions at 600 degrees C, while at 800 degrees C, larger aromatic ring systems form on the biochar, reducing defect sites.