Electrodeposited Ag atoms are allowed to nucleate on a gold surface under voltage control in an aqueous electrolyte. The kinetic features of this process are investigated under potentiostatic conditions for the cases of sub-monolayer (underpotential (UPD)) and bulk deposition (BD). For voltage steps in the UPD region, the current transients indicate the expected behavior of 2-D nucleation. In the BD region, there is evidence for mixed 2-D and 3-D phase formations. Optical second harmonic generation (SHG) is used to probe further details of these findings. According to the SHG data, in the BD region, the Ag structures that nucleate directly on Au are different from those nucleated on a pre-deposited monolayer of Ag on Au. Furthermore, in the presence of both 2-D and 3-D nucleation reactions, the two resulting types of Ag structures grow in a coupled and predominantly lateral mode with the characteristic time dependence of a diffusion controlled process. The experimental data provide an estimate for the rate of island growth in this case.