Various Pt/alumina and Pt/silica catalysts were in situ reduced and treated at room temperature under nitrogen or hydrogen in aqueous media of various pH (pH 0.7, HCl; pH 6, water; pH 10, NH4OH). Then, the catalysts were filtered, dried and activated by a direct reduction under hydrogen at 500 degreesC. The dispersions of the different samples were determined from transmission electron microscopy (TEM) and/or chemisorption measurements. On the in situ reduced Pt/silica samples, a sintering of platinum is observed under hydrogen bubbling, whatever the pH of the solution and the initial platinum particle size of the catalyst. On the Pt/alumina catalysts, the sintering is less marked and depends on the initial dispersion of the catalyst and on its particle size distribution. Thus, even a redispersion was observed in the case of the Pt/alumina catalysts of low initial dispersion treated in HCl medium (pH 0.7).