Abstract Scanning tunneling microscopy has been used to visualize dynamical rearrangements of the (2×1)O adlayer on Cu(1 1 0) during reaction with CO. Taking an initially fully oxygen covered surface at 350 K, the reaction starts at terrace edges, where vacant sites for CO adsorption exist. The reaction then proceeds almost uniformly perpendicular to these terrace edges leaving the bare surface behind. In contrast, starting with an initially fully oxygen covered surface at 470 K, the reaction results in the formation of fingerlike structures at the terrace edges. In subsequent images these fingers of bare surface area extend deep into the original adlayer along the [0 0 1] direction, i.e. along the Cu–O–Cu-chains, while the reaction along the [1 1 0] direction, i.e. perpendicular to the chains, appears to proceed uniformly exhibiting a comparatively straight adlayer boundary all the time. The observations are discussed taking into account the complex adsorbate–adsorbate interactions of the Cu–O compounds and the high mobility of these species at 470 K.