Carbon oxides such as carbon dioxide (CO2) and carbon trioxide (CO3) have long been known as important molecules in atmospheric and solid state chemical reactions. Although higher order carbon oxides have been theoretically predicted to be stable and also important, their detection has been difficult. Here, we report on the first spectroscopic detection of the Cyclic (C-2v) isomer of carbon tetraoxide ((CO4)-C-12-O-16) via its v(1) vibrational mode centered around 1941 cm(-1) via low temperature infrared spectroscopy; the identification of the (CO4)-C-12-O-18, (CO4)-C-13-O-16, and (CO4)-C-13-O-18, isotopologues supported by ab initio calculations confirms the assignments. (c) 2007 Published by Elsevier B.V.