Carbon tetroxide or Oxygen carbonate (in its C<sub>2v</sub> isomer) is a highly unstable oxide of carbon with formula . It was proposed as an intermediate in the O-atom exchange between carbon dioxide () and oxygen () at high temperatures. The C<sub>2v</sub> isomer, which is 138 kJ mol<sup>âÂÂ1</sup> more stable than the D<sub>2d</sub> isomer, was first detected in electron-irradiated ices of carbon dioxide via infrared spectroscopy.
The isovalent carbon tetrasulfide CS<sub>4</sub> is also known from inert gas matrix. It has D<sub>2d</sub> symmetry with the same atomic arrangement as CO<sub>4</sub> (D<sub>2d</sub>).