Abstract This study presents a detailed analysis of the liquid and gaseous pyrolytic products of the three principal components of biomass (cellulose, hemicellulose and lignin) using two different catalysts (HZSM-5 and its iron-modification, Fe-HZSM-5). The experiments were conducted in a semi-batch reactor under the same operating conditions for all feed materials. The results allow the determination of the provenance of aromatic compounds, which are essential components of bio-oil to be used as a bio-fuel. Transformation schemes have been proposed for each biomass component so as to better comprehend the formation of these aromatic compounds. BET specific surface area, BJH pore size distribution and FT-IR technologies have been used to characterise the catalysts, while gas chromatography-mass spectrometry (GC–MS), flame ionisation detection (GC-FID) and thermal conductivity detection (GC-TCD) were used to examine the liquid and gaseous pyrolytic products. It was firstly found that HZSM-5 favoured the decarbonylation route (production of CO), whilst Fe-HZSM-5 favoured the decarboxylation one (production of CO2) for the same feed. Then, a competition was seen to arise from the presence of the catalysts: the chemical family present in majority in the oils was the one converted by the catalysts, rather than one single family. Finally, from the transformation schemes, it was seen that even though both catalysts boosted the aromatics production, HZSM-5 produced more aromatics than its iron-modification. It was also observed that HZSM-5 formed more phenols, and hence, more coke, than Fe-HZSM-5.