The molecular and electronic structures of cucurbituril (CB) and its sulfur analogue, thia-cucurbituril (TCB), have been investigated by means of density functional theory (DFT) calculations. The HOMO-LUMO (H-L) energy gap of CB is ∼7 eV and it decreases at ∼4 eV upon replacement of its twelve oxygen atoms with sulphur atoms. The vertical ionization potential (IP) of CB and TCB are rather large, being ∼7 and 6 eV, respectively. The vertical electron affinity (EA) of CB calculated in the gas-phase is negative (about -2 eV), thus suggesting that its anion is thermodynamically unstable. On the other hand, formation of the TCB anion might be favoured in electron-donating solvents since its calculated vertical EA is close to zero. An analysis of the Kohn-Sham frontier orbitals of CB and TCB indicates that neither the HOMO nor the LUMO is degenerate.