A colourless fuming solid, SO₃, which has three crystalline modifications. In decreasing order of stability these are: α‎, r.d. 1.97; m.p. 16.83°C; b.p. 44.8°C; β‎, m.p. 16.24°C; sublimes at 50°C; r.d. 2.29; γ‎, m.p. 16.8°C; b.p. 44.8°C. All are polymeric, with linked SO₄ tetrahedra: the γ‎-form has an icelike structure and is obtained by rapid quenching of the vapour; the β‎-form has infinite helical chains; and the α‎-form has infinite chains with some cross-linking of the SO₄ tetrahedra. Even in the vapour, there are polymeric species, and not discrete sulphur trioxide molecules (hence the compound is more correctly called by its systematic name sulphur(VI) oxide).

Sulphur trioxide is prepared by the oxidation of sulphur dioxide with oxygen in the presence of a vanadium(V) oxide catalyst. It may be prepared in the laboratory by distilling a mixture of concentrated sulphuric acid and phosphorus(V) oxide. It reacts violently with water to give sulphuric(VI) acid and is an important intermediate in the preparation of sulphuric acid and oleum.