The stereochemistry of hydrogen addition to C=C bonds was studied in the diastereoselective hydrogenation of selected indene derivatives over an alumina-supported Pd catalyst. In ail cases hydrogenation occurred via bottom-side syn addition of two hydrogen atoms to the chemisorbed substrate. Hydrogenation of indenol derivatives demonstrated that methyl or ethyl groups are not sufficiently bulky to hinder the adsorption and hydrogenation of the substrate in a sterically and electronically unfavorable ("upside down") position, in which the alkyl group points toward and the OH group points away from the metal surface. The implications of this observation for understanding the mechanism of asymmetric hydrogenation over metal catalysts are discussed.