Atomic-layer epitaxy (ALE) can provide atomic-scale control of the single-crystal growth of thin semiconducting layers. Despite the widespread investigation of the epitaxial properties of this technique, few detailed studies of the fundamental surface chemistry of the process have been performed. This paper describes an overview of such an in situ study of heterogrowth using a binary reaction sequence with the precursors H2S and dimethylcadmium to grow CdS on ZnSe(100). The surface chemistry was investigated using thermal desorption spectroscopy and near-edge X-ray absorption fine structure(NEXAFS). Epitaxial growth was also characterized in the growth chamber using electron and ion surface probes.