Abstract The crystal structures of 3,5,5-trimethyl-4-oxoimidazolidine-2-thione (1) and 3,5,5-trimethyl-4-thioxoimidazolidine-2-one (2) are reported and compared with the previously determined structures of 5,5-dimethylimidazolidine-2,4-dithione, 5,5-dimethyl-2-selenoxoimidazolidin-4-one, 5,5-dimethyl-4-selenoxo-2-thione and 2-thiohydantoin. (1) is orthorhombic (a = 11.310(10)Å, b = 7.326(2)Å, c = 10.675(6)Å, Z = 4, space group Pnma; R = 0.033). (2) is orthorhombic (a = 8.349(4)Å, b = 9.719(8)Å, c = 20.337(11)Å, Z = 8, space group Pbca; R = 0.038). In all these molecules the changes of the bond lengths and angles are coherent with the influence of the different exo-chalcogen atoms bonded to C(2) and C(4). The structural data are used for a simple harmonic oscillator calculation (HOSE model) in order to evaluate the contributions of the resonating forms and to relate them with the 13C NMR chemical shifts. The changes in angles around the C(sp2) atoms are discussed in terms of the Valence-Shell Electron-Pair Repulsion (VSEPR) model.