We measured the vibrational‐rotational line profiles of 14N 16O and 15N 16O in the electronic transition spectrum by using the magnetically scanned ion line of 114Cd(II) at 2165.017 Å. The observed separation between the line of 14N 16O, Q[10(1/2)] and the line of 15N 16O, Q2[11(1/2)] was 49.6 GHz, and the half‐width of these lines was 4.4 GHz. We could observe the line profile of 15N 16O in naturally occurring NO, even though its abundance is 0.37%. Also, the ratio 15N/14N in nitric acid was determined with this technique. The lower detection limit was about 50 ppb.