The hydroformulation reaction, also known as oxo synthesis or Roelen reaction, is the atom-efficient and metal-catalyzed addition of the constituents of synthesis gas, H-2 and CO, to the double bonds of alkenes. This reaction has been discovered in 1938 by Otto Roelen in the laboratories of the chemical company Ruhrchemie AG. After filing the patent application efforts to scale up the process to technical scale have been started. However, they have only partly been successful for the plants of the Ruhrchemie AG Oberhausen and its cooperation partner I.G. Farbenindustrie AG due to the difficult circumstances. After World War II the Ruhrchemie process based on cobalt catalysts and the Ruhrchemie/Rhone-Poulenc process as the first important aqueous two-phase processes with rhodium catalysts became the most important homogenously catalyzed chemical processes.