The e.s.r. spectra of u.v.- and γ-irradiated poly-(2,6-dimethylphenylene oxide) in solution and in the solid state have been interpreted in terms of the formation of a dimethylphenoxyl radical (I) as the only major radical. In benzene at room temperature the magnetic parameters for this radical are g_av= 2·0047, a_Me(I)= 6·2 G, a_Me(II)= 5·1 G, and a_H(ring)= 1·1 G. In the solid polymer the same radical has effectively isotropic coupling of 5·7 G to the two nearly equivalent methyl groups and g⊥= 2·002₄, g_∥= 2·006₃. At 77 K, after radiolysis in air, the characteristic spectrum of peroxyl-radicals (RO₂·) was observed, but irradiation of polymer under vacuum yielded radical (I) together with at least two other radicals. Dipolar interaction between radical pairs has been inferred from the presence of a signal, at g= 4. Radical (I) was present in low concentration even before exposure to radiation, which may mean that it is an intermediate in the synthesis of the polymer.