Abstract: A process is provided for the electrochemical generation of N.sub.2 O.sub.5 in HNO.sub.3, whereby a solution of N.sub.2 O.sub.4 in HNO.sub.3 is electrolyzed. An electrolytic cell for the electrolysis is also provided, having substantially parallel electrodes in electrode compartments separated by a cell membrane. The anode is of Pt, Nb, Nb/Ta 40:60 alloy with a Pt coating. The cathode is Pt, stainless steel, Nb, Nb/Ta 40:60 alloy. The cell membrane is preferably a perfluorinated cationic exchange membrane. In use N.sub.2 O.sub.5 forms in the anolyte and N.sub.2 O.sub.4 increases in the catholyte. A suitable design of cell and its use in a single- or multi-stage electrolysis process is also described.