A study of the transport of chlorides through hardened Portland cement paste membranes is reported. The flux of chlorides was induced by migration (AC and DC fields) and diffusion. Two parallel studies were developed. A macroscopic one where the migration of chlorides was followed by Electrochemical Impedance Spectroscopy (EIS) and a microscopic one in which the natural diffusion was investigated using the scanning electrochemical microscopy (SECM). Both studies have revealed to be complementary. The local investigations performed with the SECM were successfully employed for the interpretation of the EIS data. The results have shown that the transport of chlorides occurs mainly through pores of sub-micrometre equivalent diameter. The study of the transition time has allowed to conclude that the diffusion coefficient approaches the usual value obtained in bulk solution. The macroscopic study has allowed presenting an explanation for the beneficial effect of the AC current in the transport of chlorides. A possible method for estimating the flux of chlorides from the percolation resistance is also discussed.