We present DFT calculations of the Boudouard reaction occurring under pressure over a platinum surface. The adsorbed CO molecules dissociate the O atom reacting with a CO molecule from the second layer while the carbon atom is stabilized by adsorption at the hollow site. In case of coadsorption with O-2, the carbon atom is better stabilized, forming another CO2 molecule. Then the reaction takes place at lower pressure. (C) 2003 Elsevier B.V. All rights reserved.