Voltammetry and in-situ scanning tunneling microscopy (STM) were used to examine electrodeposition of cobalt (Co) on a stationary Pt(111) electrode in 0.1 M K2SO4 + 1 mM H2SO4 + 10 mM CoSO4 (or the sulfate solution) without and with 10 mM chloride (the chloride solution). Under- and overpotential deposition (UPD and OPD) of Co resulted in reduction peaks at -0.52 and -0.74 V (vs Ag/AgCl), respectively. Hydrogen evolution was the major obstruction to Co-2+ reduction, which limited the efficiency of Co deposition at similar to 63% in both solutions. UPD of Co resulted in a highly ordered honeycomb structure in the sulfate solution, whereas that formed in the chloride solution was clearly disordered. Multilayer Co deposit formed by OPD at -0.74 V in the sulfate medium was crystalline, forming moire structures for the first eight layers, followed by pyramids made of stacked triangles. These results suggested face-centered cubic stacking of the Co deposit. Co film produced in the chloride solution was also layered, except perimeters of Co layers were mostly rugged. Distinct screw dislocations and spiral defects were seen in the Co thin films produced in both solutions.