Higher sulfur oxides are a group of chemical compounds with the formula SO_3+x where x lies between 0 and 1. They contain peroxo (O−O) groups, and the oxidation state of sulfur is +6 as in SO₃.

Monomeric SO₄ can be isolated at low temperatures (below 78 K) following the reaction of SO₃ and atomic oxygen or photolysis of SO₃–ozone mixtures. The favoured structure is:

Colourless polymeric condensates are formed in the reaction of gaseous SO₃ or SO₂ with O₂ in a silent electric discharge. The structure of the polymers is based on β-SO₃ (one of the three forms of solid SO₃) with oxide bridges (−O−) replaced randomly by peroxide bridges(−O−O−). As such these compounds are non-stoichiometric.