The formation and accumulation of Cu(I) in copper sulfate electroplating solutions are discussed. Using electrolysis, the accumulation of Cu(I) in the plating solutions was also confirmed. The accumulation rate and amount depend on the current density and electrolysis time and the dissolved gases. Cu(I) is present in the aqueous solution as a complex with polyethylene glycol and is gradually consumed by oxidation or disproportionation reactions. In the plating solutions of nanobubbled water, the formation and accumulation rate of Cu(I) was different depending on the dissolved gas species. Our results indicate the possibility of controlling the Cu(I) in plating solutions. (C) The Author(s) 2018. Published by ECS.