Both the conversion and H2O2 selectivity (or yield) in direct oxidation of H-2-to-H2O2 (using 1.7 mol% H-2 in O-2 as a feed) and also the H2O2 decomposition over zeolite (viz. H-ZSM-5, H-GaAlMFI and H-beta) supported palladium catalysts (at 22 degreesC and atmospheric pressure) are strongly influenced by the zeolite support and its fluorination, the reaction medium (viz. pure water, 0.016 M or 1.0 M NaCl solution or 0.016 M H2SO4, HCl, HNO3, H3PO4 and HClO4), and also by the form of palladium (Pd-0 or PdO). The oxidized (PdO-containing) catalysts are active for the H-2-to-H2O2 conversion and show very poor activity for the H2O2 decomposition. However, the reduced (Pd-0-containing) catalysts show higher H-2 conversion activity but with no selectivity for H2O2, and also show much higher H2O2 decomposition activity. No direct correlation is observed between the H-2-to-H2O2 conversion activity (or H2O2 selectivity) and the Pd dispersion or surface acidity of the catalysts. Higher H2O2 yield and lower H2O2 decomposition activity are, however, obtained when the non-acidic reaction medium (water with or without NaCl) is replaced by the acidic one.