The modifications of the electronic properties of cleaved GaAs (110) surfaces induced by the oxygen adsorption have been studied by contact potential difference measurement. The oxygen exposure induces acceptor and donor surface states which pin the surface Fermi level at approximately 0.45 and 0.7 eV above the valence band forn‐ andp‐doped samples, respectively. Noticeable modifications &Dgr;&khgr; of the electronic affinity are also produced. The features of the states and the possible origins of &Dgr;&khgr; are discussed.