The initial process of Cu electrodeposition on bare and (NH4)(2)S-treated p-GaAs(001) surfaces was investigated using electrochemical, reflectance, light-scattering, and in situ electrochemical atomic force microscopy (AFM) measurements. The reflectance and light-scattering measurements showed that on both the bare and (NH4)(2)S-treated surfaces, Cu deposition occurred in a cluster growth mode and the Cu clusters grew in the z-direction at a relatively negative potential where the limiting current was observed. The potential dependencies of the reflectance and scattered light intensity obtained at various potential scan rates showed that the size of the Cu clusters was smaller at the higher scan rate. The charge due to the Cu deposition at the (NH4)(2)S-treated electrode was nearly half that of the bare surface under the same experimental conditions, and AFM images showed a smaller number of Cu clusters at the (NH4)(2)S-treated surfaces, suggesting a reduction in the defect sites by the (NH4)(2)S treatment.