In Pt/ZrO2catalysts used for CO2/CH4reforming to syngas, not all accessible Pt contributes equally to the activity of the catalyst. The catalytic activity is determined by the accessibilityof Pt on the Pt-ZrO2perimeter. This is explained in terms of theCO2activation via carbonate species on the support which must bein the proximity of the Pt particles to react with the methaneactivated on the metal. The significance of the support is alsoemphasized by the low activity of Pt black and Pt/SiO2, which wereincapable of forming carbonates on the support. The perimeterconcentration of Pt/ZrO2can be changed by changing the Ptconcentration in the catalyst or by increasing the calcinationtemperature which causes sintering of the Pt particles. In contrastto the calcination temperature, the reduction temperature did notinfluence the activity although the hydrogen chemisorption capacitywas markedly decreased by increasing the reduction temperature. Assintering was excluded on basis of particle size determination fromEXAFS results a SMSI state on Pt/ZrO2during high temperaturereduction is concluded to exist. This state does not persist underreaction conditions due to the presence of adsorbed oxygen fromCO2dissociation.