Cyclopropanetrione or trioxocyclopropane is a little-known oxide of carbon with formula C3O3. It consists of a ring of three carbon atoms each attached to an oxygen atom with a double bond. Alternately, it can be thought as a trimer of carbon monoxide. This compound is thermodynamically unstable[why?][how?] and has not been produced in bulk. However, it has been detected using mass spectrometry.
It is the neutral equivalent of the deltate anion C3O32−, known since 1975. An equivalent hydrate hexahydroxycyclopropane or cyclopropane-1,1,2,2,3,3-hexol, (-C(OH)2-)3 also exists. This contains geminal hydroxy groups.