The effect of cool flame partial oxidation on the detonation sensitivity of a hydrocarbon fuel was investigated experimentally. The detonation sensitivity was quantified by measuring the run-up distance required for a deflagration to transit to a detonation wave (DDT) in a rough tube. Fuel rich pentane-oxygen mixtures at sub-atmospheric initial pressures were studied. Subsequent to the injection of the mixture into a heated detonation tube, the mixture underwent cool flame oxidation after a well-controlled delay time, dependent on the temperature of the tube. Typical delays ranged from 0.5 to 2 s (depending on temperature) and were reproducible to within one hundred milliseconds. This delay permitted the mixture in the detonation tube to be spark-ignited at various stages of the cool flame process using an igniter driven by a delay generator. The results show that increasing mixture temperature from room temperature to values below the cool flame region (below 250degreesC) resulted in an increase in run-up distance. However, as the mixture began to undergo cool flame oxidization, a significant reduction in the run-up distance was obtained (as large as 50%). The sensitization effect was found to occur only at the initial stage of the cool flame oxidation reaction. If the mixture was ignited at times long after the onset of cool flame, the mixture was found to be desensitized and the run-up distance increased. The sensitizing effect of the cool flame partial oxidation may be attributed to the presence of peroxides and free radicals associated with the initial cool flame process. However, these radical species are consumed as the cool flame reaction proceeds and the mixture becomes insensitive again. (C) 2003 The Combustion Institute. All rights reserved.