Two series of palladium-based catalysts were compared on the basis of the adsorption of CO and CO2, monitored by Fourier transform infrared spectroscopy. The first series is represented by a silica-supported palladium catalyst and by some catalysts derived from it by addition of different amounts of sodium ion, 0 less than or equal to R less than or equal to 25.6, where R is the atomic ratio Na/Pd. The second series consists of palladium catalysts supported on "model" and natural pumices. The model pumices, obtained by sol-gel techniques, are silico-aluminates containing variable amounts of sodium so that the corresponding Pd catalysts have an R value in the range 0 less than or equal to R less than or equal to 6.1. In the Pd/natural pumice catalysts, changes of the atomic ratio R’ = (Na + K)/Pd are achieved with different palladium loadings. Despite the analogous behaviour of the catalysts of both series when R = 0, the presence of increasing alkali metal ions induces different behaviour towards the adsorption of CO. On increasing R in the Na-Pd/SiO2 series there is a progressive weakening of the C-O bond to produce eventually carbonates, whereas only a decrease of the amount of adsorbed CO occurs in the Pd/model pumice series (R less than or equal to 6.1), Furthermore, only physisorbed CO bands are observed in Pd/natural pumice catalysts (R’ less than or equal to 17). Different behaviour is also noticed towards the adsorption of CO2 : the equilibrium CO2(gas) reversible arrow COads + O-ads occurs in the Pd/SiO2 series, in contrast to the Pd/pumice series where only carbonate species on the surface of the support are detected. The results are interpreted on the basis of geometric and electronic effects attributed to the different localization of the alkali metal ions in the catalysts. Decoration of palladium by the alkali metal ions is evident on increasing R in the Pd/silica series; such a decoration does not occur in Pd/pumice catalysts at any R or R’ value. Accordingly the behaviours of both series of catalysts towards the chemisorption of CO and CO2 are interpreted assuming that geometric effects are predominant in the Pd/silica series, whereas in the Pd/pumice series electronic effects are the most important, geometric effects being practically absent.