The nucleation and growth of electrochemically formed Pt adlayers on highly boron-doped, polycrystalline diamond thin-film electrodes were investigated using chronoamperometry. The experimental i - t curves measured at different fixed potentials were analyzed and compared to theoretical curves for the two limiting cases of three-dimensional nucleation and growth, as described by Scharifker and Hills [Electrochim. Acta, 28, 879 (1983)] : instantaneous and progressive. Metal phase formation at short times proceeded via a more instantaneous nucleation mechanism at lower overpotentials (eta < -700 mV) on both microcrystalline and ultrananocrystalline diamonds. However, there was a transition on both diamond types to a progressive mechanism at higher overpotentials. Based on the results, apparently, there exists a relatively constant nucleation site density on these conducting polycrystalline films at lower overpotentials, with new nucleation sites developing over time at higher overpotentials. We suppose that the initial nucleation is strongly related to heterogeneities in the density of electronic states (DOS) across a film brought about by variations in the local boron and hydrogen content more so than variations in the physicochemical properties. The shift to a more progressive nucleation and growth mechanism is consistent with an increased DOS at more negative overpotentials. Some comparison measurements are reported for glassy carbon and highly oriented pyrolytic graphite. (C) 2010 The Electrochemical Society. [DOI:10.1149/1.3430501] All rights reserved.