Abstract The kinetics of the isotopic exchange between nitric oxide and nitric acid, the basic reaction of the 15N separation by the Spindel-Taylor method, has been measured. The experimental data were obtained by introducing nitric oxide of natural abundance (0·365 per cent 15N), at atmospheric pressure into a glass contactor, containing nitric acid (0·2–3·2 M) solution labeled with 1·55 per cent 15N. The solution was magnetically stirred, at 20°C. The isotopic exchange between nitric oxide and nitric acid being a fast reaction, the influence of the rate of gas diffusion into liquid on the over-all reaction rate, was taken into account. From the experiment, it resulted that the exchange reaction between nitric oxide and nitric acid is of the second order, related to the nitric acid concentration. The rate constant was determined and a mechanism of the exchange reaction was proposed; nitrogen dioxide playing the part of a catalyst. On the basis of the rate constant the HETP for a separation column was calculated.