Acetylenes and dienes in a stream containing hydrogen, methane, C.sub.2-C.sub.6 olefins and paraffins, C.sub.2-C.sub.6 acetylenes and dienes, benzene, toluene, xylenes, and other C.sub.6+ components are hydrogenated in a downflow boiling point reactor wherein the heat of reaction is absorbed by the liquid in the reactor which produces a vapor. Besides the feed to the reactor there is a recirculating stream which is fed at a rate sufficient to ensure that the catalyst particles within the reactor are wetted. A third stream, which is provided from a second downstream liquid/vapor separator from partially condensed vapor from the first downstream liquid/vapor separator corresponding to the mass evaporated in the reactor, is fed to the reactor. The composition of the third stream controls the steady state composition of the liquid flowing through the reactor.