This study examined the influence of degradation of additive agents upon copper sulfate plating, i.e. polyethylene glycol (PEG) as a depressant, Janes green B (JGB) as a leveler, and bis(3-sulfopropyl) disulfide disodium salt (SPS) as a brightener. Trace analysis was conducted in a copper sulfate bath using high performance liquid chromatography (HPLC), which clarified the respective quantitative limits for measurement of PEG, JGB, and SPS as 10 mg/L, 0.01 mg/L, and 0.15 mg/L. The PEG molecule size and the SPS concentration decreased during electroplating. The value of throwing power was particularly lowered by the PEG degradation. Hydrogenation into N,N-dimethyl-p-phenylenediamine and methylene violet decomposed the JGB, thereby lowering the value of throwing power.