Air-stable and homogeneous gold nanoparticles (AuNPs, 1a-5a) ligated by Various secondary, phosphine oxides (SPOs), [(RRP)-R-1-P-2(O)H] = (R-1 = Naph, R-2 = Bu-t, L1; R-1 = R-2 = Ph, L2; R-1 = Ph, R-2 = Naph, L3; R-1 = R-2 = Et, L4; R-1 = R-2 = Cy; L5; R-1 = R-2 = Bu-t, L6), with different :electronic and steric properties were syrithesized via Nal31-4, reduction of the corresponding Ann S-PO complex, These reasily accessible ligands allow the -fon-nation, of well,dispersed an small nanopartides. (size 1.2-2.2 nm)) which were characterized by the use of a wide :variety of techniques, such as transmission electron microscopy, thermogravimettic analysis, U.Vvis, energy dispersive X-ray, X-ray photoelectron spectroscopy- (XPS); attenuated total reflectance Fourier transform infrared speCtroscopy. -(ATR FT-IR), and -cross polarization magic angle spinning (CF MAS) NMR spectroscopy. A pronounced, ligand -effect was found, and' CP IVIAS NMR experiments enabled 'us to probe important, differences in the polarity of the P-O 'bond of the SPOs coordinated to the nanoparticle surface depending on the type of Substituents in the ligand AuNPs containing aryl SPOs Carry - only SPO anions and are highly selective for aldehyde hydrogenation'. AuN-Ps of similar Size'made- with alkyl SPOs contain also SPOH, hydrogen bonded-to SPO anions. As a consequenee they corltain less Au(I) 'and more Au(O), as is also -evidenced by XPS. They are less selective arid active in aldehyde hydrogenation and' now show the typical activity of Ati(O)NPs in nitro group hydrogenation.