Two supported platinum catalysts (1.0 wt.% Pt/alumina (D = 51%) and 1.0 wt.% Pt/silica (D = 41%)) were 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 following various procedures. The dispersions of the different samples were determined from transmission electron microscopy (TEM) and/or chemisorption measurements. On preoxidized samples, a sintering of platinum is observed under hydrogen bubbling whatever the pH of the solution and it is higher on the silica support. An in situ reduction allows to prevent the metal accessibility loss on alumina and to restrict that observed on silica. Otherwise, the activation mode (direct reduction or calcination-reduction) can modify the platinum accessibility of the Pt/alumina catalyst treated in HCl medium.