The rotational spectrum of the molecular complex diacetyl-water has been measured by pulsed jet Fourier transform microwave (FTMW) spectroscopy. The water molecule acts as an electrofore and induces a dipole moment when combined with nonpolar diacetyl. Only one conformer has been observed, corresponding to the global minimum. Its shape is unambiguously established: the water moiety is linked asymmetrically to the organic molecule; it behaves as a proton donor to one of its oxygen atoms and interferes with the internal rotation of adjacent methyl group through a C-H center dot center dot center dot O interaction. Each rotational transition appears as a quintuplet, due to the internal rotation of the two methyl groups, which are nonequivalent in the adduct. From the analysis of the observed splittings, the V-3 barriers to the internal rotation of the two methyl groups have been determined to be 3.81(2) and 4.11(2) kJ/mol, respectively.