The electrochemical intercalation of oxygen in La2CuO4 single crystals was studied using electron paramagnetic resonance and magnetic susceptibility measurements. Chronoamperometric experiments showed that, in the intercalation process, the O2- diffusion in single crystals is extremely slow (D approximate to 10(-16) cm(2) S-1). This diffusion coefficient depends strongly on temperature and the enthalpy diffusion was found to be ca. 750 kJ deg(-1) mol(-1). Crystals under electro-oxidation for one to two weeks still show the presence of the antiferromagnetic and a superconducting phases. This was supported by the simultaneous observation of line splitting in the EPR spectra and superconducting diamagnetic signal.