[en] The unstable radical at 77 K in a gamma-irradiated single crystal of deoxycytidine 5'-phosphate was analyzed by ESR. A very anisotropic g tensor (g₁=1.9997, g₂=2.0095, and g₃=2.0773) and two approximately equal β-proton couplings with isotropic values of 8.0 G were interpreted to represent a radical of the O--CH₂--R form. The radical is formed by the breakage of the phosphate--ester bond. The calculated spin densities on H/sub() 5prime/ and H'/sub() 5prime/ in the INDO MO approximation are in agreement with the observed couplings. The lack of any hydrogen bonding in the radical is believed to bring about much smaller β-proton couplings from those previously observed and assigned to the same radical