The catalyzed direct synthesis of hydrogen peroxide (H2O2) from hydrogen and oxygen has the potential to become a lower cost and more environmentally friendly alternative to the commercially practiced anthraquinone process. However, to date safety, selectivity, and product stability challenges have prevented the commercialization of this process. The development of a commercial process will benefit from a detailed understanding of mechanism and kinetics of this challenging reaction. In this study, kinetic modeling and experiments of direct synthesis of hydrogen peroxide were carried out on Pd/Al2O3, PdAu/Al2O3, and halide modified Pd/Al2O3(X) (X = F, Cl, Br) catalysts at atmospheric pressure and at 275 K; the hydrogen and oxygen concentrations in the gas phase were measured and quantified by online mass spectrometry. Kinetic expressions for all relevant elementary reactions were proposed. The kinetic coefficients were optimized by simulating the hydrogen and oxygen consumption rates with time on stream, and the kinetics were then discussed by considering all these reactions simultaneously. Based on these results, the effects of gold and halide anions were quantified. Among all tested palladium catalysts, the Pd/Al2O3(Br) catalyst displayed the best performance due to the positive effect of the Br anion which inhibited hydrogen peroxide decomposition and direct water synthesis. The kinetic study also predicted hydrogen peroxide and water productivity during the reaction, in agreement with experimental observations.