Catalysis in a sol-gel process of titanium alkoxide was switched off to control the size of sheetlike particles of a resulting titanium oxide. The oxide growth was suspended by adding acetic acid, which regulated the catalytic activity of ammonium acetate. An ultraviolet-visible absorption band depended on the size of sheetlike particles. A lower energy region appeared for the larger size (> 1 mu m) of sheetlike particles but not for the smaller one. It is probably due to a bridging bidentate of acetate on the flat surface of titanium oxide particles.