Using a combined experimental and theoretical approach, we show that the initial oxidation of a Rh553 surface, a surface vicinal to 111, undergoes step bunching when exposed to oxygen, forming lower-index facets. At a pressure of about 10 −6 mbar and a temperature of 380 ° C this leads to 331 facets with one-dimensional oxide chains along the steps, coexisting with 111 facets. Further increase of the pressure and temperature results in 111 facets only, covered by an O-Rh-O surface oxide. Our density functional theory calculations provide an atomistic understanding of the observed behavior.