We report the results of a study of electrosynthesized CdS-HgS heterojunctions using scanning tunneling microscopy (STM), photoluminescence spectroscopy (PL), and electrochemistry. CdS thin films were grown by electrochemical atomic-layer epitaxy (EC-ALE) onto Au(lll) substrates and were terminated with a single HgS monolayer using one of two methods. The first method, which involved the chemical exchange of the terminal Cd layer with Hg2+, produced a disordered, highly polycrystalline film, as evidenced by the PL spectrum, which was dominated by CdS trap luminescence. The second method, in which the HgS monolayer was grown by EC-ALE, resulted in the formation of a high-quality heterojunction, PL measurements indicate a high degree of electronic coupling between the CdS substrate and the electrochemically deposited HgS layer in this case. These findings were confirmed by atom-resolved and micron-scale STM images.