Decomposition of H2O2 is an undesired side reaction that occurs during the direct synthesis of H2O2 over Pd/C catalysts. Experiments have been carried out to understand the influence of pressure (0.2-7.5 MPag), temperature (23-50 degrees C), pH, halide concentration, catalyst and inert gas in decomposition. A model considering these effects is proposed, achieving average deviations lower than 5%. It has been found that a pH lower than 1.8 and halide to Pd ratios over 2.2 are enough to reduce the decomposition more than 90%. Activation energies between 15.0 and 83.8 kJ/mol, with and without promoters, have been found. The influence of pressure is limited for the case of decomposition above 2.5 MPag. Decomposition increases almost linearly with the amount of the active metal catalyst. Specific decomposition rates between 6.65 and 23,295 min(-1) mol(pd)(-1) with and without promoters have been observed in this study. (C) 2011 Elsevier B.V. All rights reserved.