Abstract: In liquid-phase oxidation of hydrocarbon by air, liquid carryover in the off gas returns to the reaction system from the condeser and separator. There are three modes of carryover return, as shown in Fig. 1: backward refluxing, refluxing in situ and forward refluxing. It. is apparent that the reactant concentration distribution would vary in the reactors in series with the mode of carryover return, thus affecting the overall oxidation process. In this paper, the influence of the mode of carryover return and the reflux ratio on hydrocarbon conversion and selectivety for hydroperoxide formation are discussed. It is shown that in backward refluxing, both conversion and selectivity may be improved if the carryover consists mainly of hydrocarbon, and one obtained: Lim R→∞ Sb=1