The relationship between free radicals and the staling of beer was studied. Hydrogen peroxide was generated when beer was incubated at 60°C. When examined by electron spin resonance spectroscopic analysis, the hyperfine splitting constants of phenyl-tert-butylnitrone adducts of beer incubated at 60°C with H2O2were close to those of beer without H2O2. The production of aldehydes and the degradation of isohumulones during the incubation of beer were accelerated by the addition of H2O2or by irradiation withγ-rays. They were inhibited by the addition of ethylenediaminetetraacetic acid. It is suggested, therefore, that H2O2generated during the incubation of beer could interact with the metals present to play an important role in radical reactions in beer.