The deposition of Cu from sulphuric and hydrochloric acid solutions onto n-GaAs(100) has been studied by a combination of electrochemical and optical techniques. Reflectance-potential curves allowed Cu deposition and dissolution to be monitored in the presence of side reactions which, particularly in chloride-containing electrolytes, mask the deposition process in the cyclic voltammograms. Nucleation and growth of the Cu clusters on n-GaAs(100) has also been investigated by potential step experiments and the resulting current transients analyzed in terms of standard nucleation models. Cu was found to deposit via progressive nucleation, even onto GaAs surfaces which were covered by a ruthenium adlayer. The nucleation rate for the Ru-modified surface was clearly lower than for bare n-GaAs(100).