Abstract: A typical traditional reactor for hydrogenation consists of a tank filled with a liquid and a gas and a small particle catalyst. The reaction is carried out at high pressures and high temperatures. Lack of gas on the catalyst surface limits the velocity of reaction. Much work has been done to increase the quantity of gas on the catalyst. It has not been possible to solve this problem effectively with the techniques of today. According to the invention an extra solvent is added to the reaction mixture. By bringing the whole mixture (solvent, substrate, hydrogen and reaction products) to super-critical or near-critical state, a substantially homogeneous mixture can be obtained. By this method it is possible to control the concentration of gas on the catalyst to the desired level. The velocity of reaction is thereby increased considerably. The hydrogenation reactions principally involved comprise hydrogenation of carbon-carbon double bonds (C.dbd.