​The mechanistic interpretation of the catalytic effect of phase transfer catalysts in the selective oxidation of ethyl benzene is hampered by mass transfer effects. We demonstrate that proper experiments lead to a more correct interpretation of the role of the quaternary ammonium salt (QAS) and its counterion. Specifically, experiments unequivocally show that the main action of the counterion is to enhance physical mass transfer processes, while its catalytic effect is limited to a shift in selectivity, not activity. The QAS as a whole accelerates the induction process in the ethyl benzene (EB) oxidation by degenerate branching of its hydroperoxide (EBHP). Proper mechanistic understanding of these phenomena in QAS catalysts is especially crucial under industrially relevant conditions