Long-range coupling constants, "J(C, H), involving the formyl proton or carbon and the carbons or protons of the heterocyclic ring, were measured in the two isomeric formyl-furans and -thiophenes by employing coupled 13C NMR spectra. In furan-2-carboxaldehyde these coupling constants, obtained in solutions of different polarity, show conformational dependence and quantitatively reproduce the trend of conformer populations obtained from protonproton coupling constants. Even in thiophene-2-carboxaldehyde, 3J(C, H), where H is the formyl proton, shows that the very biased equilibrium is further shifted toward the S,O-cis form in polar media. For the 3-formyl derivatives the changes of "J(C,H) in different solvents are small, at the limit of experimental error; even so, comparison with the values measured for 4-iodofuran-and 4-iodothiophene-3-carboxaldehyde, which show an equilibrium significantly dependent on solvent polarity, enables one to verify that in these derivatives the X,O-trans form, prevailing in the equilibrium mixture, increases with increasing solvent polarity. From a least-squares analysis of the experimental values, the conformer populations in the different solvents and the coupling constants of the separate conformers were estimated. The coupling constants obtained show the effectiveness of each "J(C, H) in the conformational analysis of these compounds. Larger changes, as a function of conformer populations, are expected for 'J(C,H) relative to the formyl proton, and this proves to be the observable quantity which best demonstrates small changes in the conformational equilibrium. The thermodynamic aspects of the equilibrium for the 3-formyl derivatives are discussed and, by employing classical reaction-field theory, it is tentatively shown that the increase of the less polar X,O-trans form in polar media should be due to the fact that quadrupolar contributions prevail over dipolar effects.