Conversion of alkenes to alkanes under D-2 or H-2 (5.5 MPa cold pressure) at temperatures as low as 200-350 degrees C is promoted by thermally-activated fumed silica. There is no reaction in the absence of silica and its activity is increased by the thermal pretreatment (330 to 430 degrees C for 16 h under Ar). Stilbene (1,2-diphenylethene) treated as described with D-2 gives 1,2-diphenylethane consisting mainly of molecules containing two atoms of deuterium at aliphatic sites (H-2 NMR). At 250 degrees C diphenylacetylene is converted to stilbene-d(2) which is predominantly the cis-isomer (cis/trans > 3) showing that the hydro enation is stereoselective. Other compounds for which hydrogenation is promoted include 1-nonene and anthracene. In some cases, hydrogenation of isolated aromatic rings is observed. For example, 1,2-diphenylethane in the absence of stilbene gives 1-cyclohexyl-2-phenylethane along with some 1,2-dicyclohexylethane. It is shown that surface OH groups on the silica are converted to OD by D-2 under the reaction conditions.