Potentiostatic electrolysis was conducted in an unagitated sulfuric acid aqueous solution containing Zn²⁺ and Mg²⁺ to investigate the electrodeposition behaviors of these species and to assess the characteristics of the resulting deposits. For current densities at which Zn electrodeposition was controlled by the diffusion of Zn²⁺, the interfacial pH at the cathode increased to approximately 10, at which the hydrolysis reaction of Mg²⁺ occurs. In the absence of quaternary ammonium salts, a decrease in the Zn²⁺ concentration caused the deposition of Mg-containing compounds at potentials less noble than －1.6 V. This behavior can be attributed not only to Mg(OH)₂ formation at the cathode interface but also to decreased surface coverage of Zn(OH)₂ on the cathode. When a quaternary ammonium salt with a long-chain alkyl group was added, deposition of Mg-containing compounds occurred at potentials less noble than －1.4 V. This phenomenon is thought to be attributable to the adsorption of additives on the cathode surface, which suppresses the diffusion of Zn²⁺, reduces the amount of Zn(OH)₂ adsorption leading to the alleviation of hydrogen evolution inhibition, and increases the pH at the cathode interface. The addition of a quaternary ammonium salt with a long-chain alkyl group and with a benzyl group suggests further reduction of Zn（OH）₂ adsorption.