Tl₂O₃, is a black solid which decomposes above 800 °C, forming the thallium(I) oxide and oxygen.

The simplest possible thallium compound, thallane (TlH₃), is too unstable to exist in bulk, both due to the instability of the +3 oxidation state as well as poor overlap of the valence 6s and 6p orbitals of thallium with the 1s orbital of hydrogen. The trihalides are more stable, although they are chemically distinct from those of the lighter group 13 elements and are