Using atomic force microscopy and scanning electron microscopy, we demonstrate that a Co epitaxial film surface grown on S-passivated GaAs(001) is smoother than one grown on bare GaAs(001) and establish that the surfactant nature of sulfur plays a vital role in the formation of a smooth surface. Synchrotron radiation photoemission spectroscopy results confirm that S passivation greatly reduces the segregation of substrate atoms during film growth on a S-passivated surface. It was also found that methanol rinsing after chemical S passivation provides an even smoother surface.