A tailor-made Pd-0/K2621 catalyst was subjected to post synthesis modification via a wet treatment procedure. The aim was the understanding of the role of promoters and howif anyimprovements could be qualitatively related to the catalyst performance for the H2O2 direct synthesis. The Catalyst Wet Pretreatment Method was applied in different methanolic solutions containing H2O2, NaBr, and H3PO4, either as single modifiers or as a mixture. The catalyst was characterized by Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy. It was concluded that the modified catalysts give rise to higher selectivities compared to the pristine reference catalyst thus opening a possibility to exclude the addition of the undesirable selectivity enhancers in the reaction medium. This work provides original evidence on the role of promoters, especially bromide, allowing the formulation of a new reaction mechanism for one of the most challenging reactions recognized by the world. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 32-42, 2017