The infrared and nuclear magnetic resonance spectra of 1-acyl and 1,3-diacyl derivatives of 2-imidazolidinones and 2-imidazolidinethiones were determined, where the acyl groups were acetyl, benzoyl, or trifluoroacetyl. The frequency of the ring carbonyl stretching vibration increases with the degree of acylation and the electron-withdrawing power of the acyl group. The methylene protons of the diacylated derivatives show a single peak, the paramagnetic shift of which is similarly dependent on these parameters when the ring current effects are allowed for. This correlation between the νC=Oandfavors the conclusion that the anisotropy of the acyl carbonyl group does not affect the relative methylene proton shift in these compounds and hence they all have the same preferred configuration. This was deduced to be the planartrans,transstructure with respect to the carbonyls. In the unsymmetrical 1,3-diacyl compounds the methylene protons are also nearly equivalent, the τ-value being the average of those for the symmetrical diacyl derivatives. This, together with the changes in the νC=O(acyl), is explained by mesomeric equalization of the electronic environment of nitrogen atoms 1 and 3. The C=S bond is found to have a strong deshielding influence on all protons.