In the vapor phase, it is a diatomic molecule.
[3] Thorium monoxide molecules contain a highly polar covalent bond.
The effective electric field between the two atoms has been calculated to be about 80 gigavolts per centimeter, one of the largest known internal effective electric fields.
However, exposure of a thin film of thorium to low-pressure oxygen at medium temperature forms a rapidly growing layer of thorium monoxide under a more-stable surface coating of the dioxide.
[8] At extremely high temperatures, thorium dioxide can convert to the monoxide either by a comproportionation reaction (equilibrium with liquid thorium metal) above 1,850 K (1,580 °C; 2,870 °F) or by simple dissociation (evolution of oxygen) above 2,500 K (2,230 °C; 4,040 °F).