The vibrational exciton approach is used to answer a long-standing question concerning the infrared spectra Of CO2 particles: What causes the characteristic structure of the infrared absorption bands? The CO2 particles which are generated in a collisional cooling cell at 78 K consist of many thousands of molecules. Isotopically pure ((CO2)-C-12 or (CO2)-C-13) particles as well as isotopically mixed ((CO2)-C-12/(CO2)-C-13) particles are investigated. Combining the experimental observations with exciton calculations, the band structure in the infrared spectra can be traced back to the shape of the particles. (C) 2003 Elsevier Science B.V. All rights reserved.