The nucleation mechanisms of palladium nanoparticles from I mM [PdCl4](2-) +1 M NH4Cl electrolyte (pH = 3.4) on graphene surface were studied using cyclic voltammetry, chronoamperometry and field emission scanning electron microscopy (FESEM). It was found that due to high catalytic properties of Pd nanoparticles/graphene surface, the hydrogen co-reduction during Pd electrodeposition occurs. The kinetics of electrochemical nucleation and growth of Pd nanoparticles was also evaluated using Mirkin-Nilov-Heerman-Tarallo (MNHT) model. Regarding the simultaneous reduction of hydrogen ion on the surface of the growing Pd nuclei, a modified model developed by Palomar-Pardave et al. [M. Palomar-Pardave, B.R. Scharifker, E.M. Arce, M. Romero-Romo, Electrochimica Acta 50 (2005) 4736] was employed to estimate the kinetic parameters of the process. It was shown that the steady state nucleation rate (I-st) and the rate constant of hydrogen reduction (k(H)) increase exponentially with a potential shift to more cathodic values. Using the relation between Gibbs free energy of formation for a critical nucleus (Delta G(c)) and I-st, it is concluded that there is no thermodynamic barrier against nucleation of Pd on graphene in the electrode potential range studied in this work (E=-0.3 to -0.5 V vs. Ag/AgCl). (C) 2012 Elsevier Ltd. All rights reserved.