Alkenes are prepared by partial hydrogenation of alkynes in the liquid phase over palladium catalysts by a process which comprises A. using a fixed-bed catalyst which is obtainable by heating the preferably metallic carrier in the air, cooling, coating under reduced pressure with metallic palladium, suitable molding and processing to a monolithic catalyst elements and B. adding from 10 to 180 ppm of CO to the hydrogen used for the hydrogenation.