The standard enthalpy of formation of the acetonyl radical (CH2COCH3) was theoretically estimated using several working chemical reactions, with four variants of theoretical approaches (levels) and four extended basis sets. Our best theoretical enthalpy of formation is Delta(f)H(298)(0) (CH2COCH3) = -32 +/- 4 kJ mol(-1). This computed value corresponds to the bond dissociation energy of DH2980 (H-CH2COCH3) = 403 +/- 4 kJ mol(-1), and to the resonance stabilization energy (RE0) and the intrinsic stabilization energy (SE0) of 16.5 and 14.2 kJ mol(-1), respectively. These energies indicate a greater stabilization of the acetonyl radical than previously thought. (C) 2003 Elsevier Science B.V. All rights reserved.