The utilization of ultramicroelectrodes in studies of electrochemical nucleation has several advantages that are related to the small surface area of these systems. However, to interpret the experimental results of potentiostatic transients, it is necessary to use proper equations that take into account the mass transport by spherical diffusion. This work describes the development of such equations, considering the nucleation and growth of hemispherical nuclei controlled by diffusion towards a disc-shaped ultramicroelectrode. Two electrocrystallization models are considered here. In the first one all nuclei are formed instantaneously at the beginning of the transient, while in the other the nuclei are progressively formed during the cathodic polarization. The equations have been derived for long and short periods of time to take into account the special characteristics of certain experiments, such as those in concentrated solutions.