The variation of the position of high resolution proton magnetic resonance lines of a polar solute with dielectric constant of the solvent may be calculated on the assumption that the dipole polarizes the surrounding medium, setting up an electrostatic ‘reaction field’ which alters the distribution of the electrons responsible for magnetic shielding. The theory predicts that the shape of the solute molecule should be important in determining the magnitude of this reaction field. The magnetic shielding values measured in a range of solutions of acetonitrile, representing a rod-like molecule, and of paraldehyde, a disc-like molecule, are shown to support the ‘shape theory’ rather than the simpler treatment which assumes that all solutes are spheres. Evidence is obtained that paraldehyde is essentially only one of the many possible stereoisomers, the ‘chair’ form with all the methyl groups in equatorial positions.