The selective formation of isobutene from CO and H-2 over ZrO2 has been investigated. ZrO2 catalysts having different fraction of monoclinic phase were prepared by changing pH value in the mother solution at the precipitation of zirconium hydroxide. The rate of isobutene formation increased with an increase in the volumetric fraction of monoclinic phase in ZrO2, while those of C-1, C-2, C-3, and C-5 + were independent of the fraction. The amounts of adsorbed methoxy and formate species during the reaction and also of the surface sites with strong basicity increased with an increase in the fraction of monoclinic phase. Chemical trapping experiment showed that the amount of surface methoxy species is comparable to that of site with the strong basicity. These findings were explained by both coordinate unsaturation and stronger basicity based on the configuration of ZrO7 group in the monoclinic structure.