The regular hexagonal structure of siliceous MCM-41 with a uniform mesopore size is still maintained after 48.5 wt% ZrO2 loading. Under high loadings (greater than or equal to26.7%) of ZrO2, a very small amount of ZrO2 clusters (present as tetragonal ZrO2 phase) might be formed inside or outside the MCM-41 structure. The hydrophobicity of the SO42-/ZrO2/MCM-41 catalysts is stepwise enhanced by increasing the ZrO2 loading. The covalent S=O band 1378-cm(-1) for the SO42-/ZrO2 catalyst shifts to 1363 cm(-1) for ZrO42-/ZrO2/MCM-41 materials. Under identical ZrO2 loading, SO42-/ZrO2/MCM-41 has stronger Bronsted and Lewis acidities than PMSZM/ZrO2 (physical mixture of SO42-/ZrO2 and MCM-41). The SO42-/ZrO2/MCM-41 catalysts show high activity with a selective conversion of >95 mol% (BuOH)-O-t to MTBE at low temperatures below 160degreesC, further showing the excellent on-stream stability of these catalysts for this reaction. The rather poor activity of PMSZM/ZrO2 catalysts has been ascribed to their weak acidity.