IRRADIATION with high-velocity electrons has recently been shown to produce changes, apparently of an oxidative nature, in the fatty components of foods1–3, and further investigation of the phenomenon with butter-fat has revealed a number of interesting features. In the experiments we describe, samples 2–3 mm. thick were irradiated in the electron beam from a Van der Graaff generator and the ionization intensity was measured with a thin-walled ionization chamber4. For most of the experiments the electron energy was 1.2 MeV., and the superficial dose-rate 4 × 106 r.e.p./minute. The characteristic ‘peroxide value’ which developed was measured by a colorimetric method, using the oxidation of ferrous to ferric ions in the presence of thiocyanate5.