Halogen-substituted pyrimidines, such as 5-bromouracil and 5-iodouracil, have been used as radio therapeutic (RT) agents in cancer treatment. The radiosensitizing activity of 5-bromouracil is attributed to its reaction with electron which produce the highly reactive uracil-5-yl radical by dissociating the C5-Br bond. Using density functional methods and highly accurate Gaussian 4 (G4) theory, herein, we show that 5-bromocytosine (5-Brcyt) after reaction with electron, also, leads to the formation of cytosine-5-yl radical. However, our results show that this species can subsequently undergo a base-catalyzed tautomerization reaction to form the pi-aminyl radical followed by a second tautomerization to the thermodynamically most stable sigma-iminyl radical (cytN(center dot)). From the present theoretical calculations, we infer that the mechanism of the formation of cytN(center dot) by one-electron reduction of 5-Brcyt is straightforward and may take place in 5-Brcyt-labeled DNA in competition with the usual reactions expected for the cytosine-5-yl radical such as abstraction and water addition.