A radical observed in irradiated single crystals of cytidine, 3'-CMP, and 5'-dCMP is characterized by three anisotropic proton hyperfine couplings and is called the 3alphaH radical. The 3alphaH radical shares properties with the allyl-like radical observed in thymine derivatives. The goal of the present work is to show that the previously observed 3alphaH radicals are cytosine base radicals formed on 5-methylcytosine impurities in these crystals. Ab initio electron propagator calculations in the partial third-order (P3) approximation with the 6-311G(d,p) basis set have been used to show that these 5-methylcytosines are good hole traps, having an ionization potential comparable to that of guanine. The importance of these cytosine oxidation sites to the radiation chemistry of DNA is discussed.