High level ab initio coupled cluster (CC) and equation-of-motion (EOM) CC methods have been used to characterize the lowest doublet state of the ketenyl radical (HCCO) and its four low-lying isomers: the quartet trans- and cis-states of ketenyl, the hydroxyethynyl radical, and the oxiryl radical. These four isomers are found to be minima and to lie between 50 and 70 kcal/mol higher in energy than the ketenyl radical. (C) 2003 Elsevier B.V. All rights reserved.