Results for the electrodeposition of platinum on carbon electrodes of nanometer size are presented. It is shown that electrodes with very small electroactive areas simplify the study of the nucleation and growth mechanism involved in electrodeposition. Reducing the electroactive area of the substrate easily controls the number of nucleation sites. With the use of substrates having electroactive radii of a few nanometers, it is possible to form only one single growth center and to allow that center to grow independently. The current transient associated with the growth of such a single nucleus provides both kinetic and mechanistic information about the electrodeposition process. A mathematical formula for the current transient under combined electrokinetic and mass-transport control based on the work of Fletcher [J. Cryst. Growth 1983, 62, 505] and Kruijt et al. [J. Electroanal. Chem. 1994, 371, 13] is used to fit the transients to extract the exchange current density and diffusion coefficient of the reactants. For the Pt on carbon deposition process at low overpotentials, for which the electron-transfer steps control the overall deposition process, single nucleation is observed when the electrode is smaller than about 5 nm in size. It is found that the single nucleation and growth processes can also occur at relatively large electrodes (similar to 100 nm in size) when a high overpotential is applied so that a diffusion-controlled deposition process is established. Such a phenomenon is analyzed in terms of the depletion layer of electroactive species around the growing nucleus, and the effect that this has on the nucleation rate on the surrounding electrode surface.