AbstractThe NMR spectra of a number of 2,4‐disubstituted 1,3‐dioxolanes have been recorded and the proton chemical shifts and coupling constants derived from complete spectral analysis. Vicinal coupling constants were found to be dependent on the substituent at C‐4 and this effect is more pronounced forJ(trans)of the C‐4‐C‐5 fragment. These coupling constants also indicate a homogeneous behaviour within a series with either acisortransconfiguration, although thecisbehaviour differs from that oftransisomers. This has been interpreted in terms of definite ring conformations in substituted derivatives, while the unsubstituted 1,3‐dioxolane undergoes free pesudorotation. Calculations of coupling constants were performed by semi‐empirical MO methods, both for unsubstituted 1,3‐dioxolane and for C‐4 substituted derivatives in a large number of conformations, in order to compare calculated and experimental values; the correct order ofJ(cis)andJ(trans)for 1,3‐dioxolane is obtained only by employing energies fromab‐initioMO calculations averaged over the pseudorotation circuit. For the C‐4 substituted compounds calculated coupling constants were employed in a ‘trial and error’ process for the identification of the preferred conformations of these compounds; a set of two torsional angles for each compound was derived which allows a tentative description of the geometry of each molecule. A criticism of these geometries is given according to the evidence available on the structure