A hydrogenation catalyst prepared by combining a Group VIIIA metal compound and a hydrocarbyl-substituted silicon alumoxane and a hydrogenation process wherein said catalyst is used to hydrogenate compounds containing ethylenic and/or aromatic unsaturation. The Group VIIIA metal compound is selected from the group of compounds consisting of carboxylates, chelates, alkoxides, salts of hydrocarbon acids containing sulfur and salts of partial half esters of hydrocarbyl acids containing sulfur. Nickel, cobalt and palladium compounds are particularly preferred for use in the hydrogenation catalyst. Hydrogenation catalysts prepared with hydrocarbyl-substituted silicon slumoxanes initially exhibit less hydrogenation activity than catalysts known heretofore in the prior art and prepared with a metal alkyl of a metal selected from Groups, I, II and III. These catalysts, then afford greater control over the extent of hydrogenation, particularly when partial hydrogenation is a desired end result. Ultimately, however, the catalyst permits substantially complete hydrogenation of both ethylenic and aromatic unsaturation.