Zirconia-silica composite oxides were prepared by a sol-gel method, and the state of Zr in the silica matrix was analyzed in relation to its activity in the epoxidation of oct-l-ene with t-butyl hydroperoxide. XAFS analysis suggested that Zr in the composite oxides with low Zr content was in the coordinatively less saturated state compared with those in the oxides with higher Zr contents. Epoxidation of oct-1-ene proceeded effectively on the oxides with low Zr contents. The first step of the epoxidation was assumed to be the interaction of t-butyl hydroperoxide with the coordinatively unsaturated site of Zr.