The chemical compound 1,3,5-trioxanetrione, or 1,3,5-trioxacyclohexane-2,4,6-trione is an unstable oxide of carbon with formula C3O6. It can be considered a cyclic trimer of carbon dioxide (CO2) or as a triple ketone of 1,3,5-trioxane (1,3,5-trioxacyclohexane).
Trioxanetrione has been synthesized but is exceedingly unstable, with a half-life of approximately 40 min at −40 °C. It decomposes to give carbon dioxide.

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