We show the ability of slow electrons to effectively decompose gas phase thymidine into the sugar and thyminyl moiety, at comparable intensities. The reaction proceeds via dissociative electron attachment at energies below the threshold for electronic excitation (<3 eV). with the excess charge localized on the respective fragments. Within nucleic acid, this reaction may excise thymine from the DNA strands. Since ballistic low energy electrons are the predominant radiolytic species, the present findings represent an essential reaction in the understanding of the initial events in radiation damage of DNA. (C) 2004 Elsevier B.V. All rights reserved.