The influence of the additive thiourea in freshly made and aged solutions on copper deposition from acid sulfate solutions was studied using a Pt/Pt rotating ring-disc electrode. In fresh thiourea solutions copper deposition is retarded, but the most distinct effect of thiourea is seen during dissolution of the deposits. The intermediate in copper dissolution, Cu+, is partly complexed with adsorbed thiourea. Thus Cu+ is adsorbed at the electrode surface and the current peak at the ring due to dissolving Cu+ becomes very small. The small peak indicates that active additive is present in the deposit. Thiourea containing solutions aged for only a few days have a distinct effect on the electrode reactions. At low concentrations of thiourea (1 mg dm(-3)) a correlation between ageing and growth of the Cu+ peak at the ring is clearly seen. At high thiourea concentrations (100 mg dm(-3)) a small growth of the Cu+ peak is seen during the first few days but upon further ageing of the thiourea solution copper deposition becomes strongly inhibited. The ring current can be used as a qualitative diagnostic criterion for the concentration and state of thiourea in copper sulfate solutions.