The chemical sensitization of hydrocarbon-air mixtures to detonation initiation has been studied. Experiments were carried out in order to evaluate the effect of NO2 addition to propane (C3H8)-oxygen (O-2)-nitrogen (N-2) mixtures at ambient conditions. The run-up distance and detonation cell size were established for C3H8-O-2-N-2 mixtures without NO2 and with NO2 added as a 10 to 50% fuel additive. The results show that the addition of NO2 causes no change in either the run-up distance or the cell width, indicating that the kinetic changes brought about by the NO2 are not significant to the initiation of detonation. This result is shown to agree with kinetic models that suggest that NO2 is not very effective at promoting ignition at very high temperatures such as that characteristic of detonations. (C) 2003 The Combustion Institute. Published by Elsevier Inc. All rights reserved.