We have used atomic force microscopy to investigate the surface morphology of ZnSe films grown on GaAs(001) by molecular beam epitaxy. We report the observation of nanometer scale surface clusters 400–1200 Å in diameter and 60–200 Å in height. The clusters formex situas the result of the initial exposure of the ZnSe to atmosphere, and undergo Ostwald ripening at room temperature. Our observations, combined with the relevant literature, suggest that the cluster composition isSeO2.We propose that the oxidation of the ZnSe epilayers produces a thin layer ofSeO2which migrates to form surface clusters on a stable, zinc-related oxide surface. These findings should facilitate greater control of surface morphologies during ZnSe based device fabrication processes.