The irreversible adsorption of CO on type I and type II Pt(110) electrodes produces a structural surface modification in carbonate and sulphuric acid solutions. It has been observed that the voltammogram of the initial surface structure changes after CO adsorption and oxidation of adsorbed CO. According to the voltammetric results this modification is not due to a change from a (110)-(1 X 2) to a (110)-(1 X 1) surface structure.