To improve the understanding of the applicability of CO-FTIR spectroscopy for probing the electronic properties of catalysts, the effect of CO adsorption on the geometry of small metal particles in Pt/LTL and Pt/SiO2 catalysts with varying support acidities was determined by comparison of X-ray absorption spectra before and after CO adsorption. At room temperature, the platinum particles (first shell coordination number N > 5) supported on SiO2 were stable under CO atmosphere. By contrast, the smaller platinum particles (N < 5) in zeolite LTL reconstructed with the formation of very small platinum-CO aggregates upon admission of CO at room temperature. For both Pt/SiO2 and Pt/LTL the linear-to-bridge ratio (LB) of the CO infrared bands is a function of the support acidity/alkalinity. In the case of Pt/SiO2, the L/B ratio directly reflects the electronic properties of the catalytically active metal, since the metal particles do not reconstruct. The results show that the structure of the platinum particles in LTL zeolite, which participate in catalytic reactions is not the same as the Pt-CO aggregate analyzed by FTIR. Nevertheless, both the catalytic activity and L/B ratio are a function of support acidity.