Electrochemical impedance spectroscopy (EIS) and voltammetry are used to investigate the interaction of hydrogen with Pd and Pd3Co nanoparticles supported on a carbon-matrix. The EIS patterns are recorded in the hydrogen underpotential deposition (UPD) and overpotential deposition (OPD) regions of the voltammogram in acidic electrolyte (0.1 M HClO4) where the hydrogen interacts with palladium. The atomic ratio of hydrogen to Pd (n((H/Pd))) estimated from the H-upd region of the voltammogram of Pd nanoparticles suggests a gradual alpha-beta transition unlike the sharp transition reported with bulk Pd thin-films. However, a potential range of this transition can be estimated from the EIS patterns. The diffusion characteristics of hydrogen in the Pd lattice are absent in the EIS patterns (at mid-frequency region) of these nanoparticles due to the very small particle size. The kinetic parameters, those effectively describe the Pd-H system, are estimated from the complex non-linear least square (CNLS) fit of the EIS patterns. A maximum in the charge-transfer resistance with potential, similar to that reported with bulk Pd thin-films, is observed with both Pd and Pd3Co nanoparticles suggesting the trapping mechanism of hydrogen. (C) 2016 Elsevier Ltd. All rights reserved.