A theory is presented describing, for the first time, the temporal evolution of the fractional surface area, S(t), of 3D non-interacting nuclei growing at a rate limited by diffusion of electrodepositing ions onto substrates of a different nature. Likewise, an equation has been derived describing the potentiostatic current-time transients arising from the formation and growth of such nuclei with redox reactions occurring simultaneously on their surfaces. An equation is also proposed to describe the current due to redox reactions taking place on the surface of interacting growing nuclei. The latter is used to describe the experimental current transients recorded during nucleation and growth of cobalt at applied potentials where the proton reduction reaction occurs simultaneously with the electrocrystallization process. (C) 2005 Elsevier Ltd. All rights reserved.