Electron attachment (EA) and dissociative electron attachment (DEA) to 5-chloro uracil (5-ClU) was studied in the gas phase using a crossed electron/molecule beams technique. Besides production of a parent anion via a zero energy resonance, ion yields of nine different negative ions were observed in the electron energy range from about 0 to 14 eV. In the electron energy range from about zero to 5 eV, the formation of a transient negative ion was induced by electron attachment to the pi* resonances located at about 0.24, 1.5, and 3.6 eV leading subsequently by unimolecular decay to various negative fragment ions. Absolute partial cross sections for EA and DEA to 5-ClU were obtained from the measured ion yields using a simple calibrating method. The dominant negative ion observed in the present experiment was (C4H2N2O2)(-) (corresponding to 5-ClU minus HCl) with a mass to charge ratio of 110, followed by Cl- ion (mass to charge ratios 35 and 37), the partial cross sections being sigma(0.23 eV)=5x10(-18) m(2) and sigma(0.23 eV)=3x10(-18) m(2), respectively. The parent anion produced (5-ClU)(-) has only a cross section value of sigma(0 eV)=3x10(-20) m(2). The energetic thresholds for the formation of particular negative ions from 5-ClU in the gas phase were calculated at the G2(MP2) level of theory and compared with the experimental results. On the basis of these calculations structure and relative stability of some of the fragment ions was predicted. In the case of the two most abundant ions, their formation was observed well below the calculated electron energy threshold. The formation of these negative ions below the thermodynamic threshold is explained in terms of the high vibrational energy of the 5-ClU prior to the EA process. (C) 2003 American Institute of Physics.