A lattice-gas model is developed to describe the simultaneous oxidation and etching of Si(100) Surfaces exposed oxygen. The model incorporates nucleation of oxide islands via conversion of on-surface to back-bonded oxygen, together with an observed transformation in the shapes of just-formed islands from linear to two-dimensional. Model analysis via Kinetic Monte Carlo Simulation quantifies oxygen uptake and oxide island nucleation kinetics, including possible enhanced nucleation at step edges. Simulated etching of vicinal Si(100) surfaces reveal! that receding steps are pinned by oxide islands and transform into finger-like Structures even at higher temperatures where oxide island growth is inhibited. (C) 2008 Elsevier B.V. All rights reserved.