The energy spectrum of a ‘Fexitron’ flash X-ray generator marketed by Field Emission U.K. Ltd. and operated at 500 kV has been determined by measuring the attenuation of the single pulses through plane metal attenuators with LiF thermoluminescent dosimeters. The results have been analysed by two different iterative numerical procedures using tabulated attenuation coefficients to determine the distribution of photon fluence in the pulse. Other methods of analysis are discussed and estimates are made of the errors involved in the various methods.