A typical photonuclear experiment is considered in which the ?-ray source is the bremsstrahlung from an electron accelerator. It is assumed that the electrons incident on the radiator have kinetic energy k0 and total energy k0+� (where � is the electron rest energy) so that the bremsstrahlung contains photons of all energies k = k0. The yield A(k0) for some particular photonuclear reaction is measured as a function of the electron energy and the problem is to use the data to determine the cross section s(k) as a function of the photon energy.