Catalytic oxidation of isopropylbenzene (cumene, 210 ppm) in wet air was essentially investigated over basic NaX zeolite. Experiments were carried out in presence of water (11,000 ppm) and at a gas hourly space velocity (GHSV) of 18,000 h(-1). Over NaX, the cumene showed a catalyst activation phenomenon during the reaction time. Indeed, after 175 min of reaction at 250 degrees C the conversion of cumene increases from 45% to 100% and the CO2 yield from 30% to 95% probably because of the formation of active species mainly localized at the cavities apertures of the basic zeolite. Peroxide or peroxy radicals were proposed to be these particular active species. (C) 2008 Elsevier B.V. All rights reserved.