We describe the construction and operation of a liquid‐phase reaction cell designed in our laboratory that is attached to an ultrahigh‐vacuum (UHV) chamber equipped with the traditional surface science techniques for structure and composition analysis. The sample surface can be prepared and characterized in the UHV chamber prior to transfer in the liquid‐phase reaction cell. The transfer has been designed so that there is no loss of the UHV chamber vacuum integrity, as few parts as possible come into contact with the liquid, the surface stays clean during the transfer. The liquid‐phase reaction cell itself is designed to study liquid‐phase hydrogenation reactions at pressures up to 2 atm and temperatures up to 70 °C. A 1‐mm‐diam liquid jet with a velocity up to 6 m/s is produced by a gear pump that is incident on the sample surface to allow good mass transfer at the liquid‐solid interface. The progress of the reaction is followed by gas chromatography. We report the reaction rate data for the hydrogenation of cyclohexene on a platinum foil.