The influence of potassium addition on Pt supported on yttrium-stabilized zirconia (YSZ) was studied with FT-IR CO adsorption and CO-FT-IR-TPD, in order to understand the effect of potassium on the performance of the catalyst in reforming of mixtures of methane and ethane. Potassium modification of PtYSZ strongly influenced the conversion and rate determining steps in methane and ethane in steam reforming. Water activation is the rate determining step on PtYSZ, resulting in high surface coverage of hydrocarbon fragments during steam reforming of mixtures of methane and ethane. This led to blocking of active sites by ethane fragments and consequently low conversion of methane. If potassium is added to the catalyst, hydrocarbon activation on Pt is rate determining, resulting in low surface coverage of methane and ethane. As a result, competition effects of methane and ethane diminished on potassium modified PtYSZ, enabling simultaneous conversion of methane and ethane. The weakening of the interaction of the hydrocarbons with the Pt surface as a result of potassium addition is supported by the fact that the interaction with CO is weakened, as observed with FT-IR-TPD. (C) 2009 Elsevier B.V. All rights reserved.