The direct synthesis of hydrogen peroxide is investigated using silica supported Au, Pd and AuPd catalysts. Acid pre-treatment of the silica leads to an increase in the activity of the Pd-containing catalysts and also gives rise to a synergistic enhancement in activity when Au and Pd are combined that is absent in the untreated catalysts. The acid pre-treated mono and bi metallic catalysts show much reduced H2O2 hydrogenation activity compared to their untreated counterparts. The acid pre-treatment increases the concentration of the hydroxyl groups on the support, as demonstrated using inelastic neutron scattering, which helps to disperse the metals on the support. Pd can be dispersed effectively on silica, but Au is very poorly dispersed although this is enhanced marginally by the acid pre-treatment. The origin of the synergistic effect is discussed and is considered to be related to the incorporation of a small amount of Au within the Pd nanoparticles.