To investigate the support effect on the hydrocarbon-NO-O-2 reaction in automotive three-way catalytic reactions, operando X-ray absorption near edge structure (XANES) experiments were performed for reactions of C3H6 (or C3H8)-NO-O-2 over Pt supported catalysts under reducing conditions. gamma-Al2O3, ZrO2, CeO2-ZrO2 and La2O3 were used as the support oxides in this study. After oxidation pre-treatment, the oxidation state of the supported Pt was monitored by in situ Pt L-3-edge XANES spectra while the temperature in the reaction mixture was ramped up and the catalytic reaction was followed by measuring the concentrations of C3H6 or C3H8. NO and O-2. The operando experimental results were analyzed in terms of the catalytic reaction start-up temperature (20% HC conversion temperature) and the 50% Pt reduction temperature. A V-shape correlation was found between these two temperatures showing that an appropriate Pt reduction temperature should exist to lower the catalytic reaction start-up temperature. This result suggests that the creation of active metallic Pt sites together with the self-poisoning effect of adsorbed carbonaceous species on metallic Pt should determine the catalytic reaction start-up behavior of supported Pt catalysts. Besides the suitable Pt reduction temperature, the oxygen reactivity in the support oxide is also important for lowering the catalytic start-up temperature. The control of the Pt-support interactions to provide suitable Pt reducibility and also oxygen reactivity in oxide support are both important to achieve a lower catalytic reaction start-up temperature. (C) 2011 Elsevier B.V. All rights reserved.