Hydroxyl-terminated, poly(amidoamine) (PAMAM) dendrimer-capped platinum and rhodium nanoparticles were studied with UV−vis, Fourier transform IR, and UV-Raman spectroscopy. The adsorption bands in the region of 190−900 nm were shown to be sensitive to the electronic structure of Pt compounds in solution and in the solid state. The electron charge-transfer Pt2+ ← N band at 260 nm was attributed to mononuclear Pt2+ compounds, and resonance Raman (244 nm) established that the electron donors are amide nitrogen atoms. Multidentate coordination bonding with Pt2+ causes a strong stabilizing chelate effect, and this entropic factor hinders reduction of Pt2+ by BH4−. UV diffuse-reflectance spectra showed the appearance of new bands at 300 and 342 nm in PtG4OH deposited on mesoporous silica (SBA-15) induced by heating under reducing conditions. The new bands can be assigned to platinum clusters like binuclear Pt2+−Pt2+ and higher, dendrimer-capped analogs of platinum blue compounds. In the temperature range of 30 to 350 °C, the decomposition of G4OH begins above 150 °C with dehydrogenation of backbone. The presence of platinum and rhodium particles decreased the initial temperature of decay. In reducing conditions, the PtG4OH undergoes rearrangements of its secondary structure due to the formation of metallic platinum clusters. UV- photochemical degradation of G4OH, Pt- and Rh-dendrimer systems in ozone-rich atmosphere as well in H2 and Ar showed that effect of UV light is greater than the of ozone. The ratio of initial rates for photolysis and “dark” ozonolysis was Whv/WO3 ∼ 40.