Autoignition delay times of n-butane and iso-butane have been measured in a Rapid Compression Machine in the temperature range 660-1010 K, at pressures varying from 14 to 36 bar and at equivalence ratios phi = 1.0 and phi = 0.5. Both butane isomers exhibit a negative-temperature-coefficient (NTC) region and, at low temperatures, two-stage ignition. At temperatures below similar to 900 K, the delay times for isobutane are longer than those for the normal isomer, while above this temperature both butanes give essentially the same results. At temperatures above similar to 720 K the delay times of the lean mixtures are twice those for stoichiometric compositions; at T < 720 K, the equivalence ratio is seen to have little influence on the ignition behavior. Increasing the pressure from 15 bar to 30 bar decreases the amplitude of the NTC region, and reduces the ignition delay time for both isomers by roughly a factor of 3. In the region in which two-stage ignition is observed, 680-825 K, the duration of the first ignition stage decreases sharply in the range 680-770 K, but is essentially flat above 770 K. Good quantitative agreement is found between the measurements and calculations for n-butane using a comprehensive model for butane ignition, including both delay times in the two-stage region, with substantial differences being observed for iso-butane, particularly in the NTC region. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.