Differently from acetylacetone, hexafluoroacetylacetone is a 'rigid' molecule on the scale time of microwave spectroscopy. Its rotational spectrum, corresponding to the enolic C-s form, does not display any doubling of rotational lines, indicating high effective barriers to the internal motions. An accurate structure of the main frame of the molecule has been obtained from the rotational spectra of nine isotopic species, measured by pulsed jet Fourier-transform microwave spectroscopy. The keto form lies, according to MP2/6-311++G(d,p) ab initio calculations, about 14 kJ/mol higher in energy. (c) 2009 Elsevier B.V. All rights reserved.