Active hydrogen permeating through pure Pd, Pd 93 Ni 7 , Pd 77 Ru 7 , and Pd 77 Ag 23 membranes was applied to hydrogenation of acetylene at 373 K and atmospheric pressure. Two modes of the hydrogen-supplying route were employed, i.e. one permeating through the membranes (permeation mode) and the other previously mixed into the feed (premixed mode). The results showed that variation of H 2 flow rate leads to significant effects on the conversion of acetylene in the premixed mode but small effects in the permeation mode. This is because more activated hydrogen is directly available in the permeated mode while diffusion of hydrogen to the membrane surface and the following dissociation on it are required as the additional steps in the premixed mode