The substitution of 2-thenoyl, 5-methyl-2-thenoyl, 2-thiopheneacryloyl, 5-bromo-2-thenoyl, and 5-bromo-2-thiopheneacryloyl groups on fibrous cotton cellulose increased the radiation resistance of cellulose, as indicated by the retention of the breaking strengths of the modified fibrous celluloses at high dosages of γ-radiation, as compared with that of irradiated, unmodified fibrous cellulose. The presence of electropositive or electronegative substituents on the thiophene groups did not reduce the radioprotective effects of these groups for cellulose. Crosslinking of the cellulose thenoates in 1,3-di(4-pyridyl)-propane did not significantly reduce the radiation resistance of the thenoates. Examination of the ESR spectra of irradiated cellulose and cellulose thenoates indicated that the site of the long-lived free radicals on the irradiated cellulose molecules was not changed by the chemical modification. However, the concentration of long-lived free radicals in irradiated cellulose thenoates, at a given radiation dosage, was less than that in irradiated cellulose. The localization of energy on carbon C1 or C4 of the cellulose molecule, which leads to depolymerization and loss in breaking strength of fibrous cellulose, was decreased. The radioprotective effects of thiophene groups for cellulose were similar to those of furan and benzenoid groups.