A process for reducing the particle current in the sub-gap range of all-Nb Josephson junctions. The process results in junctions having substantially increased values of V.sub.m. In order to reduce the single particle current, the reaction between the barrier layer oxide and the counter electrode is prevented by additional process steps. After forming the tunnel barrier (4) and before depositing the counter electrode (9), the tunnel barrier surface is covered with a thin, non-continuous layer (5) of a material such as gold which is not reacting with oxygen at process conditions. Subsequently, the non-covered barrier layer surface regions (7) are strongly oxidized thereby forming an oxide layer (8) of sufficient thickness to prevent electron tunneling in these regions.