A parent acidic H Á/BEA sample characterised by crystallites with very small sizes and high external surface area has been used to prepare a series of Pt/Cs Á/BEA catalysts. The exchange three times of the highly defective HÁ/BEA in a 0.5 M CsCl solution allowed to exchange only about half of the protons, as evaluated from both chemical analysis and FTIR spectroscopy in the presence of adsorbed NH 3 . The further exchange by Pt(NH 3) 4 2' followed by impregnation with CsOH, calcination and finally reduction, led to a series of Pt/Cs Á/BEA samples with progressive increase of the caesium loading and concomitant change of the reducibility of the calcined Pt species and decrease of the size of the reduced Pt particles. The analysis of the FTIR data in the n OH and d NH ranges indicates a progressive removal of the protons. Finally, the selectivities to cracking, isomerisation, and aromatisation during conversion of n -hexane are in line with the variations of acido Á/basicity and Pt particle sizes in the samples. # 2003 Elsevier Science B.V. All rights reserved.