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As catalytic processes become more important in academic and industrial applications, an intimate understanding is highly desirable to improve their efficiency on a rational basis. Because thorough mechanistic investigations require an elaborate and expensive spectroscopic and theoretical analysis, it is a major goal to link mechanistic insights to simple descriptors, such as the reducibility, that are accessible by temperature-programmed reduction (TPR) experiments, to bridge the gap between fundamental understanding and application of catalysts. In this work, we present a detailed in-situ spectroscopic analysis of TPR results from loading-dependent VOₓ/CeO₂ catalysts, using in-situ multiwavelength Raman, IR, UV–vis, and quasi-in-situ X-ray photoelectron spectroscopy as well as in-situ X-ray diffraction. The catalyst reduction shows a complex network of different processes, contributing to the overall reducibility, which are controlled by the unique interaction at the vanadia–ceria interface. The temperatures at which they occur depend significantly on the nuclearity of the surface vanadia species. By elucidating the temperature- and vanadia loading-dependent behavior, we provide a fundamental understanding of the underlying molecular processes, thus developing an important basis for interpretation of the reduction behavior of other oxide catalysts.