Platinum particles (<1.5 nm) have been shown to behave as bases in their interaction with γ-alumina. FTIR spectra of adsorbed pyridine probe molecules showed that the acid strength of the γ-alumina was decreased by the presence of (<1.5 nm) Pt particles. Ammonium chloride treatment converts the primary Pt clusters to HxPtyClz intermediates that de-anchor from the support. Consequently, agglomeration to similar to8 nm Pt particles was observed following treatment in hydrogen at a relatively mild temperature. For the treated catalyst the IR data of absorbed pyridine show a 3 cm(-1) increase relative to the original Pt/gamma-Al2O3 catalyst, indicating a strengthening of the acidity. Changes in the Pt particle size were confirmed by FTIR spectroscopy of CO absorbed onto the Pt particles before and after treatment. Consecutive CO and pyridine probe absorption demonstrated the electronic interplay between the Pt particles and the support. Pyridine adsorption onto the gamma-alumina support of a Pt/Al2O3 catalyst pre-dosed with CO produces a nearly 40 cm(-1) lowering of the CO peak position, indicative of CO bond weakening. In the case of CO adsorbed onto a catalyst pre-dosed with pyridine, a shift in the pyridine IR spectrum was only observed from the original highly dispersed catalyst.