Triphenylphosphine (PPh₃) is demonstrated to be a useful molecular probe and tool for quantitatively characterizing the spatial distribution of noble metals on porous supports. The preparation of metal organic complexes by loading the supports with PPh₃ was performed by physically mixing the water-free catalysts with solid PPh₃ powder in nitrogen atmosphere and heating this mixture at 363 K for 20 h. Based on quantitative ³¹P MAS NMR spectroscopy, the formation of noble metal (NM) tetrakis(triphenylphosphine) complexes (PPh₃)₄NM on Pt- and Rh-containing silica and SBA-15 supports was evidenced. The large specific surface areas and mesopore diameters (3.4–4.5 nm) of these supports provide suitable conditions for the quantitative formation of (PPh₃)₄NM complexes with a molecule size of ca. 2 nm. The micropores and cages of zeolites Y (0.72 and 1.14 nm, respectively), on the other hand, are too small for the formation of these complexes. Consequently, quantitative ³¹P MAS NMR investigations of noble metal containing zeolites Y indicated that only 7% of the Pt and 6% of the Rh atoms are accessible for the complex formation with PPh₃. In other words, these noble metal quantities are located outside of the micropores and cages of the zeolite particles. The non-involved 93% of Pt and 94% of Rh atoms on the zeolites Y under study are located inside the micropores and cages of this support material, making them available for shape-selective heterogeneous catalysis.