Most studies of the electrodeposition of manganese have focused on finding the best operation conditions to obtain good recovery efficiencies. However, few studies have focused on how these variables affect the final deposition product, the formation of initial nuclei, and crystal growth. This work studies the effect of SeO2(0.14, 0.27, 0.54, and 0.81 mM) on the electrodeposition of manganese, the growth of the deposition product, and the microstructure of the crystals at different electrodeposition times. The catholyte was prepared by dissolving MnSO4 center dot H2O (0.27 M) at a pH of 3.7 and (NH4)(2)SO4(0.9 M), and the anolyte was prepared with a solution of 0.5 M sulfuric acid. A stainless steel 316 plate was used as the cathode, and a Pb-Ag alloy plate was used as the anode (both with an area of 57 cm(2)). The laboratory cell (500 mL) was partitioned with an anion-exchange membrane to prevent the transfer of ions between the compartments. Low electrodeposition efficiencies were found when using a SeO(2)concentration of less than 0.27 mM due to a high rate of hydrogen evolution and dissolution of the deposited manganese film, forming a blackish deposit (MnO2). Above this concentration of SeO2, the deposits had a compact, well-defined crystalline structure and larger grain size. The availability of selenium during the initial stage of electrocrystallization allows for the formation of the first nuclei of manganese, promoting their growth. [GRAPHICS] .