Thermochernical properties of pentane-2,4-dione (acetylacetone, Hacac) and its radical (acac) were investigated by DFT and ab initio calculations based on complete basis set (CBS) extrapolation procedures, including CBS-QB3 and a modified CBS procedure that we named CBS-QB3-tz. We provide evidence that the most stable acac radical conformer is generated by C-H bond homolysis. The enthalpy of formation of gaseous acac, Delta H-f degrees(acac,g), was estimated as -228.3 kJ/mol (CBS-QB3) and -226.7 kJ/mol (CBS-QB3-tz). Based on these results, our recommended value for Arff(acac,g) is -227 8 kJ/mol, implying that many literature values reported for metal-acac bond dissociation enthalpies in coordination complexes should be revised. (c) 2005 Elsevier B.V. All rights reserved.