The catalytic steam reforming of four different aqueous fractions of bio-oil has been carried out in a fixed bed reactor at 650 °C and atmospheric pressure using a Ni–Co/Al–Mg catalyst, employing a spatial time of 4 g catalyst min/g organics. The chemical analysis of the aqueous fractions revealed that the source of biomass (pine or poplar sawdust) and the pyrolysis unit significantly influenced the chemical composition of these liquids. Depending on their chemical composition, the initial H2 yield varied from 0.101 to 0.182 g H2/g organics and the initial CO2 yield from 0.814 to 1.28 g CO2/g organics during their catalytic reforming. Regarding catalytic stability, higher catalyst deactivation took place during the reforming of the two pine bio-oil aqueous fractions. The reforming results of the four aqueous fractions have been correlated to their chemical compositions using statistical empirical additive models developed using the Bayesian Information Criterion (BIC). This strategy enabled the identification of the chemical compounds responsible for the most significant variations observed during the reforming of the liquids. The different proportions of acetic acid and furfural in the liquids had the greatest impact on the reforming results. Acetic acid was identified as a compound with low reactivity and low coke formation. In contrast, furfural was found to have high reactivity and a high tendency to produce coke in the reforming process. Additional reforming experiments conducted with acetic acid, phenol, furfural, levoglucosan and guaiacol helped to confirm and explain the results obtained during the catalytic steam reforming of the aqueous fractions.