Secondary phosphine oxides (SPOs) can be used as ligands to stabilize catalytically active gold nanoparticles (AuNPs). These materials are active and very selective in the chemoselective hydrogenation of acrolein and other α,β-unsaturated aldehydes, but the origin of the activity remains elusive. Here, by means of Density Functional Theory we identify a cooperative effect at the AuNP-SPO interface that enables the heterolytic cleavage of the H2 molecule and its kinetically favorable concerted addition to the C=O bond (in a transfer hydrogenation-like mode) to form the corresponding allyl alcohol. From the mechanism it is possible to identify the descriptors that explain the activity for a family of SPO–stabilized AuNPs and a set of unsaturated aldehydes. The activity depends on the basicity difference between ligand and reactant and the sites available on the nanoparticle.