An electron spin resonance (ESR) signal comprised of three resolved lines of equal 19.3 Gauss separation (3×19.3 G), but unequal amplitude, is observed in x‐irradiated amorphous silicon dioxide. The radical appears exclusively in silica samples which also exhibit the methyl radical, a familiar indicator of trace carbon and hydrogen contamination. The 3×19.3 G signal is observed to grow most rapidly versus irradiation dose when methyl radical concentration is near maximum. This evidence suggests that the ESR signal is due to a radiolytic, organic radical which evolves after the methyl radical and, like the methyl radical, is trapped and stabilized in the amorphous silica network. Experimental methods of radical generation are presented, followed by discussion of models for the chemical structure of the 3×19.3 G radical. ©1996 American Institute of Physics.