The atomic reconstruction and the adsorption of oxygen on 6H-SiC (0001) surfaces have been investigated by reflection high energy electron diffraction (RHEED) dynamic rocking beam analysis. The various possible chemisorption states on the surface following the adsorption of one, two, and three oxygen molecules have been studied using periodic density functional theory. RHEED rocking beam analysis provided insights into the atomic structure of 6H-SiC (0001) 3x3 in terms of vertical and lateral displacements, as well as the initial chemisorption state of oxygen on the reconstructed surface. (C) 2003 American Institute of Physics.