Gunn-Peterson tests suggest that the Universe was reionized after the standard recombination epoch. We present a systematic treatment of the ionization process by deriving the Boltzmann equations appropriate to this regime. A compact solution for the photon spectrum is found in terms of the ionization ratio. These equations are then solved numerically for the decaying-dark-matter scenario, wherein neutrinos with a mass of the order of 30 eV radiatively decay producing photons which ionize the intergalactic medium. We find that the neutrino mass and lifetime are severely constrained by Gunn-Peterson tests, observations of the diffuse photon spectrum in the ultraviolet regime, and the Hubble parameter.