The 4-hydroxy-3,4-dihydropyrimidine-2(1H)-on-4-y1 radical (1), an elusive hydrogen atom adduct to the 0-4 position in uracil, was generated in the gas phase by femtosecond collisional electron transfer to 0-4 protonated uracil and investigated by neutralization -reionization mass spectrometry (NRMS). A fraction of radicals 1 was stable on the 5.1 mus time scale. The main unimolecular dissociations of 1 were ring cleavages and a specific loss of the hydrogen atom from 0-4, as determined by deuterium labeling. Ab initio calculations up to effective QCISD(T)/6-311+G(3df,2p) and combined density functional theory and Perturbational calculations up to B3-MP2/6-311+G(3df,2p) were used to obtain bond dissociation and transition state energies for several radical and ion dissociations. The energies were used for calculations of rate constants by RRKM and transition state theory. The dissociations observed by NRMS could not be explained by competitive reactions occurring on the potential energy surface of the ground doublet state of I and must to a large part originate from excited electronic states of I that are formed by vertical electron capture. The adiabatic ionization energy of 1 was calculated as 5.55 eV. Activation energies for hydrogen atom additions to uracil were found to inversely correlate with the addition exothermicities. The most reactive position in uracil was C-5 which was calculated to account for > 95%, hydrogen atom additions, followed by C-6 (<5% additions), while additions to O-2 and 0-4 were kinetically negligible.