A specific secondary phosphine oxide (SPO) ligand (tert‐butyl(phenyl)phosphine oxide) was employed to generate two iridium catalysts, an Ir–SPO complex and IrNPs (iridium nanoparticles) ligated with SPO ligands, which were compared mutually and with several supported iridium catalysts with the aim to establish the differences in their catalytic properties. The Ir–SPO‐based catalysts showed totally different activities and selectivities in the hydrogenation of various substituted aldehydes, in which H2 is likely cleaved by a metal–ligand cooperation, that is, the SPO ligand and a neighboring metal centre operate in tandem to activate the hydrogen molecule. In addition, the supported IrNPs behave very differently from both Ir–SPO catalysts. This study exemplifies perfectly the advantages and disadvantages related to the use of the main types of catalysts, and thus the dissimilarities between them.