This work reports thermal decomposition of cyclopentanone behind reflected shock waves over 1150 - 1590 K and 750 - 1800 Torr. Carbon monoxide is one of the main reaction products and its formation was monitored using a quantum cascade laser operating near 4.56 μm. Our results show that cyclopentanone undergoes decomposition, under the present experimental conditions, via reaction channels that produce CO almost exclusively. A recent ab initio study by Zaras et al. revealed that cyclopentanone decomposes to produce CO and ethylene by two channels: ring-opening to form a di-radical which subsequently decomposes, and concerted elimination to produce CO and C 2 H 4 directly; their predicted rate constants are much slower than literature experimental data. To resolve the rate constant discrepancy and to determine whether keto- enol tautomerism plays a significant role, we performed master equation simulations which produced results in good agreement both with the previous ab initio study and with the experimental data obtained in the present work.