In the course of our research into carbon chains trapped in matrices of molecular oxygen, we encountered an IR absorption line at 2180.4 cm(-1), which we tentatively assigned to linear C6O2.(1) In this article, we describe our attempts to confirm the assignment by partial isotopic substitution of carbon by C-13 and oxygen by O-18. A detailed analysis of the IR vibration pattern allowed the unambiguous identification of the carrier of the IR absorption as C6O2. The identification work was very much facilitated by the observation that it is possible to produce and destroy C6O2 in a controlled fashion by suitable laser exposures. With the help of this feature, most of the confusing spectral background could be removed. Two infrared absorptions at 2180.4 (v(5)) and 1817.7 cm(-1) (v(6)) and ultraviolet absorption at 252 nm were assigned to the C6O2 molecule-all figures are valid for oxygen matrices. The obtained spectral data are compared with results of quantum chemical calculations. DFT B3LYP/6-31G(d) and semiempirical PM3 methods were used for geometry optimization and calculation of vibrational frequencies. CIS and TD-DFT were used to calculate the electronic absorption spectrum.