A difficulty for the electrochemical deposition is the high negative electrolytic potential of the Mn+2/Mn degrees pair in aqueous solutions producing water reduction at any pH value; to resolve this problem it is necessary to use selenium dioxide (SeO2) as additive. However, there is little information about the selenium mechanism presenting discrepancies. Founded on linear voltammetry, cyclic voltammetry, chronoamperometry test, and impulses of constant potential and thermodynamic analysis, are experimental evidences of the reaction mechanism to explain the selenium action; from here the originality of this work. It is demonstrated that two reactions that occur in the selenium mechanism at - 0.3 and - 0.6 V, the first for the selenium deposition and the second for the selenium dissolution. On the active sites of the cathode the specie of selenium reduced at - 0.3 V is the ion hydrogen selenite (H2SeO3-), while at - 0.6 V the deposited selenium is reduced to selenide.