A polydomain internal structure was evidenced by means of High Resolution Transmission Electron Microscopy (HRTEM) in palladium nanoparticles of two 5% Pd/SiO2 catalysts prepared by impregnation technique. Such poly-domain structure biased the line broadening analysis of the XRPD patterns, from which highly underestimated average diameters were obtained in comparison to those measured by means of Transmission Electron Microscopy (TEM).In both the investigated catalysts, a Pd/CO average chemisorption stoichiometry close to 2 was found, in good agreement with previous results obtained for Pd/C and Pd/gamma-Al2O3 catalysts. The consistency of this result, in cases with different particle sizes and different support materials, suggests that such average stoichiometry is of general validity for palladium, when chemisorption measurements of CO are made using pulse flow technique.