Abstract A novel kinetic Monte Carlo simulator “ARC” was developed to investigate hydrogen diffusion process in metals and alloys. The results of the hydrogen diffusion coefficients in palladium, silver, and Pd 1− x Ag x ( x ≤ 0.5) are in excellent agreement with experimental results. Two different diffusion sites, Pd-rich diffusion site and Ag-rich diffusion site, are found to be necessary to explain diffusion behaviors of hydrogen in palladium–silver alloys. Detailed analysis clarified that hydrogen atoms preferentially diffuse through the palladium-rich region of palladium–silver alloys at low silver concentrations, and that the hydrogen diffusion process at low silver contents is dominated by the jumps in palladium-rich regions. The migration barrier of hydrogen from Pd-rich diffusion site to Ag-rich diffusion site is estimated to be between 0.40 and 0.45 eV.