The microscopic process of the formation of oxides on metal surfaces is barely known or understood. Using density-functional theory we studied the oxidation of Ru(0001): from the initial oxygen adsorption, subsequent O incorporation into the metal, aggregation of sub-surface islands, to the transition to the oxide film. Along the atomistic pathway several metastable precursor configurations are identified. It is argued that their properties and the metastabilities in the surface-oxide formation process will have important consequences for the discernment and molecular modeling of catalysis. (C) 2002 Elsevier Science B.V. All rights reserved.