Selective hydroformylation of styrene to iso-aldehyde, 2-phenylpropionaldehyde in higher than 90% yield has been achieved catalyzed by silica-supported poly-gamma-aminopropylsiloxane-ruthenium-cobalt bimetallic complex catalysts (Si-NH2-Ru-Co) in the presence of acetic acid. The Ru/Co mole ratio, the amount of acetic acid and total pressure greatly influenced the reaction. The catalyst was stable and could be used repeatedly without any remarkable changes in activity and selectivity.