By the use of guanidinium compounds, more specially surface active guanidinium compounds--that is to say guanidinium compounds with an anionically active radical--by themselves or in connection with guanidinium carbonate as auxiliary reagents the hydrogenation of heavy oils may be promoted, there being a greater penetration of the hydrogen into the liquid phase and an activation of normally-used metal hydrogenation catalysts, this making possible a marked decrease in the least reaction pressure, which in the prior art is about 150 bar.