Stoichiometric laminar premixed H2/O2/Ar flames stabilized on flat burners at pressures 1 3 and 5 atm were studied using molecular beam mass spectrometry. Mole fraction profiles of all flame species including H2 O2 H2O H2O2 H O OH and HO2 were measured. A decrease in the peak mole fractions of H O and OHradicals and an increase in the peak mole fractions of HO2 and H2O2 with pressure was observed. Two detailed kinetic mechanisms for H2 combustion which were validated against new experimental data reported presented that both mechanisms reproduced well the mole fraction profiles of H2O2 H2O and H O OH radicals in the flames. These experimental results can help in further development and improvement of the future kinetic models of H2 combustion at elevated pressures.