Carbon trioxide (CO<sub>3</sub>) is an unstable oxide of carbon (an oxocarbon). The possible isomers of carbon trioxide include ones with molecular symmetry point groups C<sub>s</sub>, D<sub>3h</sub>, and C<sub>2v</sub>. The C<sub>2v</sub> 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 ().
Carbon trioxide can be produced, for example, in the drift zone of a negative corona discharge by reactions between carbon dioxide (CO<sub>2</sub>) and the atomic oxygen (O) created from molecular oxygen by free electrons in the plasma. Another reported method is photolysis of ozone O<sub>3</sub> dissolved in liquid CO<sub>2</sub>, or in CO<sub>2</sub>/SF<sub>6</sub> mixtures at , irradiated with light of 253.7 nm. The formation of CO<sub>3</sub> is inferred but it appears to decay spontaneously by the route
with a lifetime much shorter than 1 minute. Carbon trioxide can be made by blowing ozone at dry ice (solid CO<sub>2</sub>), and it has also been detected in reactions between carbon monoxide (CO) and molecular oxygen (O<sub>2</sub>). Along with the ground state C<sub>2v</sub> isomer, the first spectroscopic detection of the D<sub>3h</sub> isomer was in electron-irradiated ices of carbon dioxide.