Ab initio calculations of potential energy surfaces in conjunction with the RRKM-Master Equation theoretical approach have been employed to evaluate temperature- and pressure-dependent total and product specific rate constants and product branching ratios for unimolecular thermal decomposition of 2,4-cyclopentadienone C5H4O and for the C5H4O+H and C5H5+O reactions. The formation of the cyclobutadiene+CO products via a ring contraction/CO elimination mechanism is shown to be the prevailing channel for the unimolecular decomposition of C5H4O. The unimolecular reaction is found to be relatively slow, but decomposition of cyclopentadienone can be greatly facilitated through bimolecular encounters with H atoms. The C5H4O+H reaction is predicted to be fast, with rate constants ranging from 4.6 x 10(-12) to 1.8 x 10(-1) cm(3) molecule(-1) s(-1) at T = 500-2500 K and finite pressures. Cyclic C5H5O intermediates formed after the initial H addition undergo ring openings by beta-scissions and then decompose the major and the minor reaction products. The calculations predict that thermal decomposition of the ortho and meta C5H5O radicals as well as pyranyl nearly exclusively forms the C4H5+CO products, whereas decomposition of hydroxycyclopentadienyl C5H4OH predominantly produces cyclopentadienone+H. The C5H5+O reaction is shown to proceed by barrierless oxygen addition to the ring followed by fast H migration, ring opening, and dissociation to C4H5+CO. The C5H5+O rate constant is calculated to be close to 1 x 10(-10) cm(3) molecule(-1) s(-1) and to be pressure-independent and nearly independent of temperature. Modified Arrhenius expressions for rate constants for all considered reactions at the high-pressure limit and at finite pressures are generated for kinetic modeling. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.