The aim of this study is to investigate the effect of CO2 on catalytic hydrotreatement of gas-oil to evaluate the potential of catalytic co- hydroprocessing of petroleum fractions with liquid biomass. A total of six experiments were conducted under low (330 degrees C), mild (345 degrees C) and severe (360 degrees C) operating conditions employing straight run gas-oil as the petroleum fraction feedstock. During three of these experiments CO2 was artificially added to the reaction system to assess its impact on the associated catalytic reactions over a commercial CoMo/Al2O3 catalyst. The dedicated experiments revealed a suspending role of CO2 on hydro- desulfurization (HDS), hydro-denitrogenation (HDN), and saturation reactions, which is attributed to the subsequent hydrogen partial pressure reduction of the reacting system caused by the competitive water gas-shift and methane reforming reactions.