Deactivation of TiO2 is hampering practical implementation of photocatalytic alternatives for energetically intensive selective oxidation processes. In the present study, humidification of the air stream is demonstrated to be a solution to this problem for well-defined photocatalysts, such as Solaronix TiO2, in selective oxidation of cyclohexane. Water establishes enhanced product desorption and stimulates decomposition of deactivating surface carboxylate species, as clearly demonstrated by ATR-FTIR spectroscopy. For high surface area TiO2, such as Hombikat UV100, water molecules adsorb too strongly, and addition of water vapor to the air stream is detrimental to catalyst performance. (C) 2010 Elsevier Inc. All rights reserved.