Dual argon plasmas ignited by one direct current power source are used to treat an aqueous solution of hydrogen tetrachloroaurate-(III) trihydrate (HAuCl4 • 3H2O) which is contained in an H-Type electrochemical cell. The solution contained in one cell acts as a cathode, and in the other as an anode. Experiments are carried out to directly visualize the formation process of gold nanoparticles (AuNPs) in separated cells of the H-Type electrochemical reactor. The results and analyzes suggest that hydrogen peroxide and hydrated electrons generated from the plasma-liquid interactions play the roles of reductants in the solutions, respectively. Hydrogen peroxide can be generated in the case of the liquid being a cathode or an anode, while most of hydrated electrons are formed in the case of the liquid being an anode. Therefore, the reduction of the AuCl4- ions is mostly attributed to the hydrogen peroxide as the liquid acts as a cathode, while to the hydrogen peroxide and hydrated electrons as the liquid acts as an anode. Moreover, the pH value of the solution can be used to tune the formation processes and the final form of the AuNPs due to its mediation of reductants.