Polycrystalline platinum electrodes emersed from H2SO4 and HClO4 solutions were transfered to ultrahigh vacuum for study by thermal desorption spectroscopy (TDS). The double layer was generated at potentials between 0.05 and 2.75 V. Our interest was focused on the surface coverage with water, oxygen and sulphate as a function of the applied electrode potential. The desorption of O2 from polycrystalline Pt starts 500 mV earlier (at 700 mV/RHE) than the value obtained earlier on Pt(100). The chemical state of thin films generated for 30 s at positive potentials seems to be a mixture of hydroxides and oxides. Whereas water desorption signals increase in parallel with those of oxygen, the sulphate coverage decreases with increasing potential. The growth of anodic layers induced by a square-wave cycling method was monitored by thermal desorption of water and oxygen as well as by cyclic voltammetry. In contrast with thin anodic layers, anions are incorporated into thick layers generated in H2SO4 and HClO4.