The poisoning effect of Cl on the activity of Pt-supported catalysts for CO, methane, and ethane oxidation has been investigated by kinetic studies and in situ IR and controlled atmosphere EXAFS spectroscopies. Catalysts containing 1.5% Pt/Al2O3 were prepared by incipient wetness from H2PtCl6 and Pt(NH3)(4)(NO3)(2) precursors. The reduced catalysts have similar dispersion (0.8) as estimated by H-2 chemisorption. The Cl-free catalyst was 10 times more active than the Cl-containing catalyst during CO and ethane oxidation. Addition of HCl to the Cl-free catalyst rendered its activity identical to the catalyst prepared from Cl-containing precursors. The presence of Cl also affects the activity of 2% Pt/SiO2 catalysts, but to a lower extent. On the Cl-free oxidation catalyst, Pt-Pt and Pt-O bonds were detected using EXAFS, suggesting that the reduced metal particles are not fully oxidized under the reaction conditions. Additionally, chemisorption of CO by the oxidized catalyst indicates that a portion of the reduced Pt atoms is exposed to the reactants. On the Cl-containing catalyst, there are also Pt-Cl as well as Pt-Pt and Pt-O bonds. The later catalyst, however, does not chemisorb CO, indicating that there are no reduced surface Pt atoms. The effect of Cl poisoning on the oxidation activity of Pt supported on silica is similar to that on alumina. IR results show that chlorine significantly reduces the amount of CO adsorbed on metallic Pt sites. At low temperature there is little CO adsorbed on the Cl-containing Pt/silica catalyst, while at higher temperature the amount of adsorbed CO increases, likely due to reduction of the oxidized surface. The catalyst activities correlate well with the amount of reduced surface sites, and a model is proposed to explain the mechanism of chloride poisoning, which is shown to occur mainly by site blocking.