The safe operation of hydrocarbon liquid-phase oxidation by air or oxygen requires the knowledge on the flammability of hydrocarbon/oxygen mixtures in both the vapor space and vapor bubbles. The latter is of particular importance in situation where pure oxygen is used as the oxidant as most bubbles are expected to be flammable and explosive. New experimental findings are presented for ignition and explosion in cyclohexane liquid under oxygen oxidation conditions. A bubble column is constructed and fitted with multiple igniters. Experiments were performed at liquid temperatures between 373.15 and 423.15 K under various flow rates of pure oxygen. Two drastic different ignition and explosion behaviors were observed. The first is a typical bubble explosion from the direct ignition of the flammable bubbles in the liquid. The explosion occurs immediate following the ignition and do not produce significant energy that endanger the system. The other is a remote, delayed ignition and explosion in the vapor space that can produce significant overpressure and endanger the system. The explosion is attributed to the ignition of flammable vapor space by active free radicals from cyclohexyl hydroperoxide decomposition. A mechanism is proposed for the remote, delayed ignition to occur in the oxidation system. It is concluded that explosion in an oxidizing, bubbly liquid is not only a likely scenario but also a severe scenario, and cyclohexane oxidation should not be carried out directly with pure oxygen and without any inerting. (c) 2005 Elsevier Ltd. All rights reserved.