Ketonization of biomass-derived propionic acid was investigated at 270-350 degrees C over CexZr1-xO2 mixed oxides with varying Ce/Zr ratios. The acidity of CexZr1-xO2 increases as Zr content is increased. The density of the strong base sites is reduced with increasing Zr content, while the density of the medium-strength base sites is maximized on Ce0.5Zr0.5O2. Diffuse reflectance infrared spectroscopy of propionic acid adsorption indicated that increasing Zr content in CexZr1-xO2 favors monodentate adsorption over bidentate adsorption. The reaction rate varies with the composition of CexZr1-xO2. The intrinsic reaction rate can be correlated with the medium-strength acid/base ratio. The rate maximizes at a medium-strength acid/base ratio of , similar to 1.07 on Ce0.1Zr0.9O2. This value is close to 1, indicating that the balanced medium-strength acid-base sites play a crucial role in the ketonization. The balanced acid- base property influences propionic acid adsorption, enolate formation, and C-C coupling and therefore affects the ketonization rate.