A comparison of theoretical and experimental STM images for the 2 x I superstructure of oxygen adsorbed on Ru(0 0 0 1) is Presented. The analysis of STM images unveils the interplay between surface geometry and electronic effects. We use first-principles methods to calculate the electronic structures of both tip and sample which are then used to evaluate the tunnelling current in Bardeen's approximation. Our simulated images compare well with the measurements and allow us to identify the atomic-scale features seen in the images. (c) 2005 Elsevier B.V. All rights reserved.