During co-hydroprocessing of petroleum fractions with liquid biomass, carbon monoxide (CO), carbon dioxide (CO2) and water (H2O) constitute some of the main by-products which take place due to the oxygen content of biomass. This study focuses on determining the H2O and CO effect on HDT reactions, as there is a certain controversy associated with their role. H2O and CO are artificially added during gas-oil catalytic hydrotreatment to investigate their influence in the potential biomass (i.e. lipids, bio-oil, tallow oil) catalytic co-hydroprocessing with fossil-based fractions for the production of hybrid, fuels. Based on the experimental results, it is clearly inferred that the H2O role is neutral on heteroatoms' removal. On the contrary, CO has a suspending role not due to the resulting hydrogen partial pressure reduction, but rather to the inhibition of the catalyst active sites. (C) 2015 Elsevier B.V. All rights reserved.