The structure of the ionosphere of a CO-rich comet is computed using two different models. The first one, the photochemical model, assumes that the dissociation and ionization of cometary neutrals and ions are due to photoionization and photodissociation by solar uv radiation together with dissociative recombinations and ion-neutral reactions. The second one, the internal source model, also incorporates the ionization and dissociation effects of an electric current dischanging through the inner coma. The generation of this current has been discussed in earlier papers. It is concluded that the internal source model can explain qualitatively the basic morphology of the ionospheres of CO-rich comets such as Humason (1962, VIII) and Morehouse (1908, III), whereas the photochemical model cannot. The main aim of this paper is not so much to provide accurate numerical estimates as to draw attention to a process which may very well dominate the structures of cometary ionospheres.