The destruction of free radicals by oxygen diffusion into x‐irradiated Lexan, [OC6H4(CH3CCH3)C6H4O(CO)]n, has been studied by means of optical and electron paramagnetic resonance measurements. Lexan, which is normally amber, appears uniformly green immediately after x‐irradiation, but in a few hours, begins to bleach from the edges toward the interior, and a visible amber border forms about a green core. By direct measurements of the rate of recession of the boundary, it was found that the phenomenon is due to O2 diffusion and that the process obeys an Arrhenius expression (between 0 and 130°C.) with an activation energy of 0.31 e.v. (±3%). It has been shown that Zener's equation for the diffusion “frequency factor,” D0 = γS2v exp {ΛBE/kTm}, is useful in the study of diffusion in polymers even though it was derived for the case of atomic diffusion in metallic crystals. In the spectral region studied (0.3–0.7 μ) there are two bands (near 0.40 and 0.34 μ) present in the irradiated polymer which are not present in the unirradiated material. The center producing the absorption at 0.34 μ is stable over a long period. The absorption at 0.40 μ decays in a few days at room temperature when oxygen is present. At 250 Mcycles/sec. and 9000 Mcycles/sec. there was a single EPR absorption with a width of 13 ± 3 gauss, and at 250 Mcycles/sec., a g value of 2.012 ± 0.007. Measurements of changes in the spin concentrations with irradiation dose and sample age (measured from the instant of removal from the x‐rays) showed that the color center which absorbs near 0.40 μ is paramagnetic. The radiation G value (at 40 k.v. and 18 kr/min.) for the paramagnetic centers was estimated to be about 5 spins/100 e.v. by comparison of the Lexan EPR spectrum with that of a DPPH spin standard. A simplified rate equation which was consistent with the decay data may be written: n = K'n/tα, where n is the average spin concentration as a function of time t and ≃ 1/2. The t−α factor can be interpreted in terms of a diffusion‐limited supply of oxygen in the interior of irradiated Lexan.