Cathodic modification of platinum surfaces leads to the formation of iono-platinic phases ([Pt(n-), M+, MX]), which involves the insertion of cations and salts into the platinum electrode. This process was investigated at the local scale by in situ observation of surface electrochemical processes by atomic force microscopy (EC-AFM) techniques as a function of the salt and the injected charge, with special attention about the process reversibility. AFM images recorded in solution after the cathodic modifications of well-defined platinum surfaces [epitaxial platinum deposit on (100) MgO substrate] show drastic modification on the morphology of the surface, confirming previous ex situ studies. The amplitude of the modifications directly depends on both the nature of supporting electrolyte and the quantity of charge injected into the platinum. As long as the injected charge remains small enough to maintain the adhesion of the Pt deposit onto the MgO substrate, the process was found to be fully reversible. Indeed, impressive morphology changes occur under the cathodic treatment (formation of [Pt(n-), M+, MX]) but the initial geometry is totally recovered after reoxidation of the iono-platinic phase. This cycle of reduction-reoxidation can be performed several times without any significant alteration of the recovered surface and of its structural characteristics. It is suggested that the modification starts at the interface solution platinum surface and then its insertion into the platinum surface.