In order to improve thermal stability and catalytic oxidation activity of platinum-alumina cryogels prepared by one-pot sol-gel reactions and subsequent freeze drying, molar ratio of aluminum sec-butoxide, nitric acid, and water used for the sol-gel processing was optimized. It was found that the sintering of platinum at high temperatures was more remarkably suppressed at ASB:HNO3:H2O = 1:0.17:87, at the ratio of which higher CO oxidation activity was also observed. It was suggested that the suppression of sintering of the metal was attributed to smaller sizes of mesopores obtained at this ratio, retarding transfer and aggregation of platinum.