Limited knowledge of ionospheric processes of the outer planets and practically no information on the upper atmosphere of Neptune result in a range of possible ionospheres for Neptune. Various cases are investigated in order to establish a theoretical framework of the ionospheric structure against which the radio occultation measurements can be evaluated. The peak electron densities could range from 103 cm-3 to 106 cm-3 depending on ion loss processes and particle ionization processes. The scale height could also range from 300 km to 2000 km depending on the exospheric temperature. Although various assumptions must be made, we predict that an auroral ionosphere with large electron densities (~105 cm-3) and a large scale height (~2000 km) will be observed by the ingress radio occultation measurement, and that a compressed ionosphere with small electron densities (order of 103 cm-3) will be seen during the egress measurement. ¿ American Geophysical Union 1989