ABSTRACTEffects of ionizing radiation on beef pigments were studied by determining the absorption spectra of myoglobin solutions and the reflectance spectra of radiation‐sterilized beef slices. Bovine and ovine oxymyoglobin (MbO2) and met(ferri)myoglobin (metMb) were extracted, purified, and treated with gamma radiation. Increasing dose and temperature of gamma radiation produced increasing shifts in characteristic peaks and progressive decreases in the Soret bands of these pigments. The total color difference (▵E), computed from tri‐stimulus values of reflectance spectra of radiation‐sterilized beef, showed similar dependence of color on radiation dose and temperature. Re‐irradiation of beef allowed to brown in air caused the unstable red pigment to re‐form. The presence of oxygen in the container during radiation‐sterilization reduced the red color formation. The results show that ionizing radiation reduces the heme iron of the brown pigment of cooked meat (globin myohemichromogen) to an unstable red pigment (globin myohemochromogen), which, upon exposure to air, reverts to the original ferric (b