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Abstract: A parent acidic H-BEA with crystallites very small in size and high external surface area was used to prepare a series of materials loaded with increasing Cs+ contents by firstly ion-exchange and then impregnation with CsOH solutions. The monitoring of the ion-exchange process by chemical analysis and by IR spectroscopy in presence of CO or NH3 reveals that a relevant amount of Brønsted acid sites in dehydrated H-BEA is related to framework Al sites that, in aqueous solution, turn into partially extraframework Al species unable to act any longer as sites of cationic exchange. This limits the exchange capacity in solution and higher levels of ion-exchange are attained by subsequent impregnation and calcination. A possible explanation for such a behaviour is proposed. The formation of carbonates by adsorption of CO2, monitored by IR, confirms that the basic character induced on framework oxygen atoms by exchange of H+ with Cs+ is significantly weaker than that reached upon Cs-overloading. For the latter, the strong basicity is related to the presence of Cs2O-like nanoparticles (also detected by EXAFS), dispersed within the zeolite pores (as shown by pore volume and TEM/EDX measurements). IR spectroscopy of adsorbed CO shows that Cs+ as countercations or as surface sites of occluded Cs2O-like species exhibit a similar Lewis acid strength. Noticeably, in Cs-overloaded BEA, pairs of Cs+ sites (formed by two countercations and/or one countercation and a Cs+ at the surface of Cs2O-like particles) are present, where CO can be adsorbed in a head–tail form, producing a distinct ν CO band at 2145cm−1. [Copyright &y& Elsevier]