H2O2 formation directly from H-2 and O-2 on Pd particles in water has proven to proceed via two different kinetic regimes defined by the operating H-2/O-2 ratios using kinetics, isotopic experiments and variation of particle sizes under conditions of kinetic control. These kinetic regimes exhibit different rate equations, kinetic isotope effects and structure sensitivity, while a transition among regimes occurs as the surface reactive intermediates changing from nearly uncovered to saturated with chemisorbed hydrogen (H*) with the increasing of H-2/O-2 ratio. On nearly bare Pd surfaces, irreversible H-2 dissociation limits the rates of H2O2 formation and H-2 conversion, thus, rates increase proportionally with H-2 pressure but independent of O-2 pressure. On H* saturated Pd surfaces, H-2 dissociation approaches to chemical equilibrium and the H* mediated HOOH* formation step becomes the sole kinetically relevant step, therefore, rates become insensitive to H-2 pressure but are linearly proportional to O-2 pressure. The first-order rate coefficients for H2O2 formation become a single-valued function of the operating H-2/O-2 ratio. (C) 2019 Elsevier Inc. All rights reserved.