The biradical radiationless decay mechanism of the excited electronic state structure of the cytidine, 5-fluorocytidine, and deoxycytidine nucleosides are studied at CIS and coupled cluster CC2 levels with the cc-pVDZ basis set. Nucleoside radiationless decay pathways are compared with those of the isolated bases. For the ground state geometry, cytidine and deoxycytidine as well as isolated cytosine have S-1 pi(H)pi(L)* states. In moving towards lower energy configurations, this doorway pi pi* state crosses over to a biradical excited state, which in turn intersects the ground electronic state and is responsible for the S, So internal conversion. Qualitatively, the radiationless decay mechanisms of cytidine nucleosides are similar to the isolated cytosine base. (c) 2006 Elsevier B.V. All rights reserved.