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We have investigated the relationship between the reducibility of the support and the catalytic activity of supported Pt on a series of catalysts supported on ceria–zirconia mixed oxides. The supports were prepared by co-precipitation with varying Ce/Zr ratios. X-ray diffraction analysis indicated that, depending on the Ce/Zr ratio, solid solutions of cubic Ce_xZr_1−xO₂ can be formed. The reducibility of the supports was determined by X-ray photoelectron spectroscopy (XPS). After reduction at 773K in hydrogen the fraction of reduced cerium (i.e. Ce³⁺) was found to vary with the Ce content, exhibiting a maximum at a composition Ce_0.5Zr_0.5O₂. A good correlation was found between the reducibility and the catalytic activity. It was found that the conversion of methane and CO₂ obtained on the different catalysts after 22h on stream went through a maximum as a function of Ce content in the support and that maximum …