Palladium absorbs hydrogen exothermically. Under the industrial conditions for hydrogenation reactions over palladium catalysts, palladium particles should be working as palladium hydride α'-PdH_x (x > 0.6), which suggests the enrollment of absorbed hydrogen in the hydrogenation reaction. In this article, after the energetics, kinetics, and microscopic mechanism of the hydrogen absorption and release processes on palladium surface is reviewed, the recent research on the role of absorbed hydrogen on the alkene hydrogenation reaction on palladium is briefed.