Carbon oxides of the form COn (n = 2-8) have long been known as important molecules in atmospheric and solid state chemical reactions. Here, we report on the first infrared spectroscopic detection of the cyclic (C-s) isomer of carbon hexaoxide ((CO6)-C-12-O-16) via its v, vibrational mode centered around 1876 cm(-1) under matrix isolation conditions; the identification of the (CO6)-C-12-O-18, C-13(16) O-6, and (CO6)-C-13-O-18, isotopologues supported by ab initio calculations confirm the assignments. We also discuss possible formation routes of this molecule. (c) 2007 Elsevier B.V. All rights reserved.