The electrochemically active surface area (ECSA) of metal-oxide supported platinum catalysts as obtained from hydrogen underpotential deposition (H upd ) and from carbon monoxide stripping experiments was investigated. It was demonstrated that both methods fail to give meaningful values of the ECSA if they are performed in the conventional way as known for pure Pt and carbon supported Pt catalysts, respectively. For both methods, the reason for this failure is the lack of a correct baseline for the integration of the associated charges. It was found that the cyclic voltammogram recorded in CO saturated electrolyte gives an improved baseline for the H upd analysis. For CO stripping, a novel baseline method was developed by performing a "CO stripping simulation" (COSS) experiment in CO-free electrolyte. The first cycle of this COSS-experiment is an improved baseline for the integration of the CO stripping peak, since possible support reduction/oxidation currents can be accounted for. With these modifications, H upd and CO stripping voltammetry can be used for metal-oxide supported platinum to yield true, reproducible and consistent values for the ECSA. In recent years, an increased attention in the research on oxygen reduction reaction (ORR) catalysts for polymer electrolyte fuel cells (PEFC) has been focused on metal-oxide supported platinum catalysts.1 The reason for this interest is the potentially higher stability of the metal-oxide support in the oxidative electrochemical environment of a PEFC cathode in comparison with the carbon support of standard Pt/C catalysts for ORR. The resistance of the support toward corrosion is especially important under load cycling and start/stop conditions with cathode potentials reaching values up to 1.5 V vs. the adjacent electrolyte.2,3 State-of-the-art cathodes made of Pt nanoparticles supported on high surface area carbon suffer from severe corrosion at potentials above 1.1 V due to the oxidation of the carbon support. [4][5][6] The consequent detachment of Pt catalyst nanoparticles or their dissolution leads to a strong degradation of PEFC performance. 7,8 Replacing the carbon support by suitable metal-oxides in an oxidation state which is thermodynamically stable at PEFC cathode potentials can help to mitigate this degradation mode on the materials side. Furthermore, it is well established in heterogeneous catalysis that metal-oxide supports can possibly influence the intrinsic activity of the supported Pt catalyst toward ORR due to so-called strong metal-support interactions, SMSI.9-12 Thus, metal-oxides may offer a way to kill two birds with one stone: Enhancing the cathode stability and enhancing the cathode catalyst kinetics toward the ORR.In order to experimentally assess both the stability and the ORR activity properties of metal-oxide supported Pt catalyst, the determination of the electrochemically active Pt surface area (ECSA) is inevitable due to its widespread use as an important descriptor of the state of the degraded catalyst and electrode. Measuring the ECS...