Abstract: This invention relates to improved hydroformylation reactors of the "loop" type comprising two parallel vertical tubes, one of said tubes having a supply tube at its base and acting as the ascending branch and the other of said tubes acting as the descending branch, and means connecting said tubes to one another at their top and bottom ends, the improvement comprising a tubular loop connecting said tubes at their upper ends, and an evacuation orifice for synthesis gases and excess liquid phase from said tubular loop, said orifice having a smaller diameter than the internal diameter of said tubular loop.