The presence of alkali cations in zeolite framework provides peculiar basic properties that influence the characteristics and reactivity of supported metal particles. Particularly, the addition of increasing amounts of cesium in Pt/Na-BEA and Pt/Cs-BEA catalysts, by CsOH impregnation, leads to changes of the oxidation state and reducibility of Pt species present in the zeolite microporosity. For similar contents of impregnated Cs, the metal dispersion is higher in Pt/Cs-BEA than in Pt/Na-BEA series, showing that both the amount of Cs excess and the nature of the compensating cation affect the characteristics of the Pt species. In the case of Cs-overloaded PUCs-BEA, the size of Pt nanoclusters is below 10 angstrom. The very small Pt nanoparticles in reduced Cs-overloaded Pt/Cs-BEA are particularly active in aromatization of n-hexane. (c) 2005 Elsevier B.V. All fights reserved.