Magnesium deposition and dissolution processes in ethereal Grignard salt solutions using tetrahydrofuran as the solvent and Grignard salt which included RMgCl, RMgBr, and R2Mg (R = methyl, ethyl, butyl, or benzyl) are reported. Mg dissolution and deposition in these solutions are basically reversible with an overall mass balance close to zero for complete cycles. In prolonged Mg deposition processes, the mass accumulated per mole of electron transferred was similar to 12 g/mol, the equivalent weight of magnesium. However, Mg deposition-dissolution is not a simple two-electron process, but involves adsorption-desorption processes of species such as MgR . and/or MgX2 (X = Cl, Br). These adsorption processes lead the Mg electrodes to develop high impedance upon storage in these solutions (up to hundreds of thousands of Omega cm(2)). However, this passivation is not stable, and breaks down as the electrochemical processes proceed.