Carbon trioxide (CO₃) is an unstable oxide of carbon (an oxocarbon). The possible isomers of carbon trioxide include ones with molecular symmetry point groups C_s, D_3h, and C_2v. The C_2v state, consisting of a dioxirane, has been shown to be the ground state of the molecule. Carbon trioxide should not be confused with the stable carbonate ion (CO²⁻
₃).

Carbon trioxide can be produced, for example, in the drift zone of a negative corona discharge by reactions between carbon dioxide (CO₂) and the atomic oxygen (O) created from molecular oxygen by free electrons in the plasma. Another reported method is photolysis of ozone O₃ dissolved in liquid CO₂, or in CO₂/SF₆ mixtures at −45 °C, irradiated with light of 253.7 nm. The formation of CO₃ is inferred but it appears to decay spontaneously by the route

2 CO₃ → 2 CO₂ + O₂

with a lifetime much shorter than 1 minute. Carbon trioxide can be made by blowing ozone at dry ice (solid CO₂), and it has also been detected in reactions between carbon monoxide (CO) and molecular oxygen (O₂). Along with the ground state C_2v isomer, the first spectroscopic detection of the D_3h isomer was in electron-irradiated ices of carbon dioxide.