The effect on nitrogen chemical shifts of 14N 15N isotopic substitution has been examined by 1H{ 14N} and 1H{ 15N} heteronuclear double resonance. For the four molecules (CH 3) 4N +I −, C 6H 6CH 2NC, CH 3ONO 2 and (CH 3) 2NNO 2 and their 15N anologs the ratio of 15 N 14 N resonance frequencies is R = 1.40275695 ± 8 × 10 -8. It is concluded that there is no significant isotope effect on the nitrogen shieldings and that R gives directly the ratio γ ( 15 N) γ( 14 N) .