The reverse leakage current of p-n junctions usually has a non-negligible surface peripheral component. Experimental data presented for silicon junctions reveal that published evidence in favour of dominance of the bulk component may be uncertain. It has been found that the surface leakage current is the major component not only in junctions with high carrier lifetime but also in junctions with low lifetime, as for example those bring subjected to electron irradiation. The experiments show that a variation of reverse current with the square or cubic root of the applied voltage is manifest when the surface component is dominant and a temperature dependence like that of the intrinsic carrier density is encountered. Yet the bulk component is the major part in junctions with a high density of recombination centres, but the junction maximum permissible voltage is controlled by the surface component. Except for high temperatures an activation energy below 1.0 eV has been found. Furthermore it is shown that the surface component may significantly contribute to a deviation of the forward characteristics from the Shockley ideal law. In spite of a significant progress in manufacturing technology the surface leakage current is due to the semiconductor-dielectric interface defects. The presence of the surface reverse leakage current can lead to instability during device operation and finally junction degradation. For this reason the operation temperature of most silicon devices is limited to 175-200 degrees.