The rotational spectra of five isotopologues of the molecular adduct 1,1,1-trifluoroacetone-water have been assigned using pulsed-jet Fourier-transform microwave spectroscopy. All rotational transitions appear as doublets, due to the internal rotation of the methyl group. Analysis of the tunneling splittings allows one to determine accurately the height of the 3-fold barrier to internal rotation of the methyl group and its orientation, leading to V(3) = 3.29 kJ . mol and angle(a,i) = 67.5 degrees, respectively. The water molecule is linked to the keton molecule on the side of the methyl group through a O-H center dot center dot center dot O hydrogen bond and a C-H center dot center dot center dot O intermolecular contact, lying in the effective plane of symmetry of the complex.