A complete proton ENDOR study (∼-160°C) of a fairly stable radical, produced by gamma irradiation at room temperature in single crystals of urea-oxalic acid (1 : 1), is presented. The radical was assigned to a protonated carboxyl anion, in spite of somewhat unusual experimental proton hyperfine coupling tensors. The assignment could be justified by considering delocalization of the spin distribution in the π-system extending over the entire oxalic acid molecule. A satisfactory spin density distribution map was determined. The experimental tensors are compared with the theoretically reconstructed dipolar tensors, using the procedures of McConnell and Strathdee, also taking into account a small negative spin density contribution due to spin polarization of the OHβ bond of the intramolecular protons. The inclusion of latter contribution was found to be necessary as they are comparable to the usual direct contributions. Combined with the results of the previous study in urea-oxalic acid (2 : 1), the reasons f...