Oxidation of diphenylamine to diphenylnitroxide in neutral, K-exchanged, X-type faujasite zeolite (K-X) has been observed. The oxidation, as assessed by the intensity of the diphenylnitroxide electron paramagnetic resonance (EPR) spectrum, is favored in K-X in comparison with diphenylamine doped into X-type faujasite zeolites exchanged with Li, Na, Rb or Cs. Similar experiments with analine gave no detectable free radical product. The weakly allowed Delta m(x)=+/-2 half-field peak of a triplet state EPR spectrum with root D-2+3E(2)/g(e) beta(e)=1200+/-150 G is observed. The triplet spectrum is attributed to molecular oxygen probably immobilized at a cation site in the zeolite. We proposed that O-2 also binds to Lewis acid sites in the zeolite and hydrogen bonds to diphenylamine. This diphenylamine-O-2 complex rearranges to form diphenylnitroxide and the OH. radical. The mechanism implies that the OH. is somewhat stabilized by the zeolite. The temperature dependence of the diphenylnitroxide EPR spectrum indicates a restricted degree of molecular motion in the zeolite cages.