A modified form of ferrous sulphate dosemeter is described, which retains the accepted advantages of the Fricke dosemeter and possesses the following additional properties: (a) it determines the dose in water; (b) the response is independent (to within +or-1.5%) of effective photon energy over the range 24 keV to 50 MeV and, therefore, independent of the shape of X-ray spectrum; (c) the low concentration of sulphuric acid reduces the possibility of introducing impurities from the source.