The reactions over HZSM-5 catalyst of a number of model compounds were studied in a fixed-bed micro-reactor operating at 3.6 WHSV, atmospheric pressure and temperature range 330-410 degrees C. The compounds were propanoic acid, methyl ester of acetic acid, 4-methylcyclohexanol, cyclopentanone, 2-methylcyclopentanone, methoxybenzene, ethoxybenzene, phenol and 2-methoxy-4-(2-propenyl)phenol. These compounds represented the acid, ester, alcohol, aldehyde and ketone, ether and phenol chemical groups, which were identified as the main components of a bio-oil obtained by the high pressure liquefaction (HPL) of aspen poplar wood. Also, the reactions of a synthetically prepared volatile feed and bio-oil volatiles over HZSM-5 catalyst were investigated. The objective was to understand and identify the reaction steps involved in the HZSM-5 conversion of the bio-oil. The intent was to obtain a reaction pathway which could be used for modeling the conversion of the bio-oil. Based on the results, two reaction pathways were proposed. It was observed that the conversion of the bio-oil was complex and involved a number of reactions. These reactions could be categorized as: 1. Primary reactions. These include cracking and deoxygenation. 2. Intermediate or secondary reactions. These include secondary cracking, oligomerization, olefin formation and cyclization. 3. Terminal or terminating reactions. These include alkylation, isomerization, disproportionation, polymerization and condensation reactions.