The present work has provided new fundamental information on the effects of aging (mileage, km) of a commercial Pd-Rh (9:1, w/w) three-way catalyst (TWC) on (a) the H₂ chemisorption, (b) the redox properties of washcoat material, and (c) the dynamic oxygen storage and release properties of TWC. Hydrogen chemisorption in the 25–400°C range was found to decrease with increasing aging of TWC. On the other hand, H₂ chemisorption performed at 200 and 400°C resulted in a significantly larger amount of chemisorbed hydrogen (exceeding the monolayer value based on the noble metals) compared to that obtained after chemisorption at 25°C. This is due to the onset of a hydrogen spill over process in the 200–400°C range. The features of H₂-TPD spectra were found to strongly depend on the aging of TWC. Hydrogen chemisorption at 25°C followed by TPD after a given pre-treatment of the catalyst surface might be considered as a procedure for a good estimate of the metal dispersion of a commercial Pd-Rh TWC. The redox properties of the oxygen storage components of the washcoat of a commercial Pd-Rh TWC were found to drastically change with increasing aging of TWC. It was found that the oxygen storage capacity (OSC) of the TWC investigated decreases significantly with catalyst aging in the 0–56,000km range and in the temperature range of 500–750°C. In the case of fresh TWC, the presence of 20ppm SO₂ or 10% CO₂ in a 1.5% O₂/He gas mixture used for oxygen storage (oxidizing gas) was found to result in a large decrease in the amount of dynamic OSC measured by alternating switches between oxidizing and reducing feed gas compositions. The shift in time of the peak maximum of the transient response of CO₂ obtained during the switch O₂/He → He → CO/He (t) can be ascribed to the alteration of the kinetics of the oxygen back-spillover process and not to the kinetics of CO oxidation on the metal surface.