The electron spin resonance of gamma-irradiated single crystals of methoxycarbonylcholine picrate hemihydrate, C7H16NO3+. C6H2N3O7-. 1/2H(2)O has been observed and analyzed for different orientations of the crystal in the magnetic field. The crystals have been investigated between 70 and 350 K. The spectra were found to be temperature independent and the radiation damage centers are attributed to -(C) over dot (O) over bar OCH3 and -CH2CH2O(C) over dot radicals. The g and hyperfine coupling constants were found to be almost isotropic with an average, g = 2.0060, a(H), = 4.4G for -CH2CH2O(C) over dot, g = 2.0050, a(H2) = 3.5G for -CH2CH2O(C) over dot and g = 2.0045, a(H) = 3.5G for -(C) over dot (O) over bar OCH3. These values indicate a long-range coupling between the unpaired electron and H protons.