In the purification process of automobile exhaust, existing water plays an important role as an oxidant, which converts CO and hydrocarbons (HCs) by the water-gas shift (WGS) and the steam reforming (SR) reactions, respectively, especially at high temperatures. Meanwhile it is major component of the exhaust which can affect significantly the thermal stability of the three-way catalyst. Activity experiments were carried out close to the real operation conditions (GHSV, concentration, etc.) with a Pd/Ce0.67Zr0.33O2 catalyst supplying information on the CO and C3H8 oxidation reactions in feedstream formed by different reactant combinations. The obtained results showed that the activity of the CO and C3Hs oxidation was promoted by the addition of steam due to the WGS and SR reactions. The WGS and SR reaction were competitive under oxygen-lean conditions. The kinetic analysis was considered for WGS and SR reactions.