The absolute value of the yield of ferric ion on the irradiation of ferrous sulphate in 0.80  Nsulphuric acid by γ-rays has been determined using the Bragg–Gray cavity principle. Sources of error in the measurements were reduced as much as possible. The yield obtained, 20.8 ± 0.3 ferric ions per 100 ev. absorbed, agrees well with Miller's value of 20.7 molecules per 100 ev.