Gas/liquid reactions are carried out in a downstream reactor in which the gas having a bubble diameter of 0.1 to 4 mm is injected into the liquid flowing downward at an empty tube velocity of 0.14 m/s to 0.5 m/s. The proportion by volume of liquid to gas should be greater than 8:1 and the residence time of the gas at least 6 second per meter of reactor length.