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Nickel-based heterogeneous catalysts are economical alternatives to traditional Pt or Au-based materials, but their specific activity and stability fall short of these precious metals. Many questions remain regarding the active Ni phase for CO oxidation and how the supporting oxide affects its activity and stability under operating conditions. We demonstrate that CeO2aerogel-supported Ni(OH)2nanoparticles (NPs) show superior activity compared to Ni(OH)2NPs supported by commercial, nanometric CeO2powder or TiO2aerogels. Ex situ X-ray photoelectron spectroscopy and X-ray absorption near-edge spectroscopy reveal that intimate contact between Ni(OH)2NPs and the bonded nanoparticulate CeO2network prevents the conversion to NiO seen with larger aerogel-supported aggregates of Ni(OH)2NPs and unsupported Ni(OH)2NPs. The best stability and activity per mole of Ni is achieved at 2.5 wt % Ni(OH)2on the CeO2aerogel, which converts 95% of CO at 200 °C and maintains activity after 12 h of continuous reaction under dry or humid feedstreams. Computational analysis indicates that CO has far lower adsorption energy on Ni(OH)2compared to NiO, resulting in less poisoning of the active surface sites.