The basic isotopic exchange reaction is responsible for the separation of ISN in the Nitrox system that between gaseous nitrogen oxides and aqueous nitric acid with a single stage separation factor ~= 1.055 for 10M nitric acid, at 25 ~ and atmospheric pressure takes place. In order to know what happens in ~5N separation at higher pressure, when the isotopic transport between two phases is improved, a stainless steel laboratory experimental plant with a 1000 mm long • 18 mm i.d. column, packed with triangular wire springs 1.8• mm 2, was utilised. At 1.5 atm (absolute), and 2.36 mlcm -2 .min -l flow rate HETP was 7% smaller than at atmospheric pressure and 1.5 times smaller flow rate. HETP at 3.14 mlcm-2.min -I flow rate and 18 atm is practically equal with that obtained at atmospheric pressure and 2 times smaller flow rate. The operation of the ISN separation plant at 1.8 atm (absolute), instead of atmospheric pressure, will permit doubling of the 10M nitric acid flow rate and of lSN production of the given column.