The mechanism of enantiodifferentiation in the hydrogenation of alkenoic acids over cinchona-modified Pd has been investigated using the tiglic acid → 2-methyl-butanoic acid transformation as test reaction. Application of simple derivatives of cinchonidine, modified at the (C-9)–OH and/or the quinuclidine nitrogen, proved that both functional groups are involved in the enantiodiscriminating step. Addition of a strong base (1,8-diazabicyclo[5.4.0]undec-7-ene, DBU) to tiglic acid prior to hydrogenation revealed that one cinchonidine molecule interacts with a dimer of tiglic acid on the metal surface. Ab initio calculations corroborate the existence of an energetically favored acid dimer–cinchonidine intermediate stabilized by hydrogen bonding, involving both the OH and the quinuclidine nitrogen of cinchonidine.