Abstract The annealing of Cu-adatom configurations on a Cu(100) surface was studied by means of low-energy ion scattering (LEIS) experiments and numerical simulations. In the LEIS experiments, the adatom configurations, with a total coverage of ∼ 10% of a monolayer, were created by low-temperature ion-beam irradiation of the Cu(100) surface. The adatom signal was monitored as a function of increasing temperature. It was observed to decrease slowly for surface temperatures above 140 K, due to diffusion of adatoms to steps, and the formation of adatom clusters. Some of the adatom configurations were observed to be stable up to a temperature of 200 K. Dynamical Monte Carlo (DMC) simulations were performed to study the diffusion of adatoms and adatom clusters, and the formation and stability of clusters. The energy barriers for diffusion, needed in the DMC simulations, were obtained from Finnis-Sinclair atom-embedding calculations. The simulation results were compared with the experimental results, and qualitative agreement was found.