The lattice gas model with hcp and fee sites is used to study the O/Ru(0 0 0 1) adsorbate system. With interactions obtained from density functional theory its phase diagram is calculated using Monte Carlo simulations. Good qualitative agreement is obtained between experiment and theory; better quantitative agreement occurs if the interactions are scaled by a factor of 3/4. A (root3 x root3)*R30degrees structure at a coverage of 2/3 ML is predicted. (C) 2002 Elsevier Science B.V. All rights reserved.