Cyclopropanetrione or trioxocyclopropane is a little-known oxide of carbon with formula C3O3.

Alternately, it can be thought as a trimer of carbon monoxide.

This compound is predicted to be thermodynamically unstable, dissociating to carbon monoxide,[1] and has not been produced in bulk.

However, C3O3 molecules, provisionally assigned to either cyclopropanetrione or its open-chain analog •(CO)3•, have been detected using mass spectrometry.

[2] It is the neutral equivalent of the deltate anion C3O32−, known since 1975.