[en] Lanthanide oxysulfides normally have the composition M₂0₂S (with M = La - Lu). The composition, M₂0S₂, remained unknown for a long time. With Tb₂0S₂ the first example accessible at ambient pressure has now been obtained through the oxidation of Na_0.25TbClH_0.75 with sulfur and apparently ubiquitous oxygen impurities as a by-product. The crystal structure is built up from 0^2- centered [Tb³⁺]₄ tetrahedra that are connected through a common edge to form dimers [Tb₆0₂]¹⁴⁺. A more convenient route to Tb₂0S₂ is the crystallization from a NaCl melt starting with appropriate mixtures of Tb₂0₃ and Tb₂S₃