AbstractIn order to separate inductive and lone pair effects on geminal and vicinal coupling constants in a stereochemically well‐defined system, the1H NMR spectra of phenylcyclopropane (1),N‐methyl‐2‐phenylaziridine (2), styrene oxide (3) and 1,1‐dimethyl‐2‐phenylaziridinium fluorosulfonate (4) were compared. In D2O the heterocyclic ring protons of 4 were split into an ABX pattern which gaveJ(cis)= 8.5,J(trans)= 7.4 andJ(gem)= −4.8 Hz (signs consistent with INDOR results). From the small solvent effects onJ(vic) determined from 4‐d1, it was concluded that J(gem) is −5.0 ± 1.0 Hz in methylene chloride. The absolute values for the coupling constants for 1 and 4 provide a measure of the inductive effect of the ring hetero group onJ.Values ofJ(gem) for 2 and 3 deviated from those predicted on the basis of the above inductive effect, suggesting lone pair contributions toJ(gem) ofc.+5.5 Hz per lone pair. With this estimate it was possible to predict accurately theJ(gem) values for 2‐t‐butyloxaziridine and 1‐t‐butyldiaziridine. The values ofJ(cis)andJ(trans)for 2 and 3 likewise suggested a contribution of −2.5 Hz toJ(cis)and −2.7 Hz toJ(trans)per lone pair. The present results suggest that the major factors causing positiveJ(gem) values in epoxides and aziridines are increasedscharacter to the CH bonds and lone pair effects, while the so‐called electronegativity effect actually operates in the opposite direction todecrease J(gem). Also, the unusually lowJ(vic) values of epoxides relative to cyclopropanes are now seen to be due more to negative lone pair contributions than to the el