Acid-base properties of silica-supported ytterbium oxide catalyst, loading amounts of which were 25 mu-mol-8.4 mmol Yb g(SiO2)(-1), were investigated by TPD experiment and ol-pinene isomerization. The relations among acid properties, structures, and catalytic properties are discussed. The catalytic activity depends on loading amounts of ytterbium and pre-treatment temperatures, and reached the highest when loading amount was 3.4 mmol g(SiO2)(-1) and pre-treatment temperature of 1073 K. No crystalline phase was detected with XRD technique. By XAFS spectroscopy, Yb atoms adjacent to Yb were found to be absent, but the presence of Si atoms adjacent to Yb was observed in all the catalysts. XRD and XAFS analyses revealed that Yb atoms are supported on SiO2 in a highly dispersed form as YbO6 octahedra. Solid acidity is concluded to be due to strong interaction between a YbO6 octahedron and a SiO4 tetrahedron, and the specific activity per Yb-Si unit was independent of loading amounts. When loading amounts of Yb were excess to form two-dimensional ytterbium oxide overlayer, total turn over numbers were drastically reduced.