The jet-cooled rotational spectrum of uracil has been investigated using laser ablation molecular beam Fouriertransform microwave (LA-MB-FTMW) spectroscopy. The quadrupole structure originated by the two 14Nnuclei of uracil has been completely resolved and assigned. This provides a definitive method to establish thepyrrolic or pyrimidinic nature of the N nuclei and allows us to conclude that the observed tautomer of uracilin the gas phase is the diketo form. From the rotational constants of the isotopic species detected, a structureof uracil has been determined.