This work describes the influence of quaternary ammonium compounds [(R4N)X-+(-)] in the yield of ethylbenzene hydroperoxide (EBHP) during the liquid-phase oxidation of ethylbenzene (EB) with molecular oxygen. Reaction rates were affected not only by the type of R4N+ and X- ions but also by their concentrations in the reaction medium. In the presence of structurally different quaternary ammonium bromide salts, ethylbenzene conversion increased because the ammonium salt favors the interaction with the EBHP. The type of the X- anion also modified the time-course of the EBHP and EB conversion profiles, although this effect was less notorious in the former; the highest EBHP concentration was reached with tetramethylammonium iodide and the lowest one with the hydroxide salt. Both EB conversion and EBHP yield decreased strongly upon reducing the concentration of the catalyst from 10(-3) to 10(-4) M and this effect was even more marked for the iodide salt than for the bromide counterpart. (c) 2005 Elsevier B.V. All rights reserved.