Electron attachment (EA) to the DNA bases thymine (T) and cytosine (C) was studied in the electron energy range from about 0 to 9 eV using a crossed electron/molecule beams technique. As uracil the dominant negative ions formed via EA reactions to T and C are (T - H)(-) (corresponding to T minus H, with a mass to charge ratio of 125) and (C - H)(-) (corresponding to C minus H with a mass to charge ratio of 110). The respective partial EA cross-sections for (T - H)(-) and (C -H)(-) could be determined yielding peak values of sigma (1.05 eV) = 1.2 x 10(19) m(2), sigma (1.54 eV) = 2.3 x 10(-10) m(2), respectively. (C) 2003 Elsevier B.V. All rights reserved.