The temporal generation of anionic platinum-carbonyl complexes in platinum ion-exchanged zeolites X and Y by reductive carbonylation at 10(5) Pa and 363 K is monitored by in situ UV/VIS and FTIR spectroscopy. A monomer [Pt-3(CO)(6)](2-), exhibiting bands at 318 and 456 nm in the UV/VIS spectra and at 1790 and 2025 cm(-1) in the FTIR spectra, is the only platinum/species formed in NaX. The monomer as well as oligomers are generated in NaY, where the formation of the latter species is due to the stronger acidity in the NaY as compared to NaX. The decomposition of the complexes results in the generation of Pt clusters of the size less than or equal to 1 nm.