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Abstract: The potential energy surface of C2H2O2 for HCCO+OH bimolecular reactions and the unimolecular decomposition of hydroxyketene (HOCH C O) are investigated employing high-level quantum chemical methods. Variable reaction coordinate transition state theory is used for the high-pressure limit rate constant calculation of the main reaction channels and RRKM-based multiwell master equation is used to calculate the pressure dependent rate constants and product branching ratios of these channels. The predicted rate constants are in good agreement with the limited experimental data available in the literature. The product distribution analysis shows that the association/decomposition of HCCO+OH to the formation of CO+ 1HCOH and CO+ 3HCOH channels are dominant in the whole temperature range of 500–2000K below 1atm, whereas at higher pressure and low temperature, the association reaction producing 3CHCOOH becomes competitive. For the unimolecular decomposition of hydroxyketene, the formation of CO+ 1HCOH channel is found to be predominant over a wide range of temperatures and pressures. Rate constants of these reactions and thermodynamic parameters for species involved in these reactions are also provided. [Copyright &y& Elsevier]