Modification of 15 wt% H4SiW12O40SiO2 with Na+ ions was investigated. Na-modified H4SiW12O40/SiO2 showed higher selectivity for isobutene oligomerization in the preferential oligomerization of isobutene in an equimolar mixture of isobutene and 1-butene than the unmodified one and the selectivity increased with an increase in the Na+ ion content. The highest selectivity (similar to 97%) was achieved by using Na3HSiW12O40/SiO2. The activity decreased with an increase in the Na+ ion content. Unmodified H4SiW12O40/SiO2 had two kinds of acid sites with different acid strengths (medium and strong acid sites) on the outermost surface as revealed by temperature-programmed desorption of benzonitrile. However, the strong acid sites were eliminated by modification with alkaline metal cations: for example, Na3HSiW12O40/SiO2 had a negligible amount of strong acid sites. Thus, we concluded that the absence of strong acid sites on the outermost surface is the reason for the high selectivity. Li3HSiW12O40/SiO2 and K3HSiW12O40/SiO2 also exhibited high selectivity for isobutene oligomerization with activities comparable to that of Na3HSiW12O40/SiO2. (C) 2010 Elsevier B.V. All rights reserved.