Various models for the calculation of mass spectra, the critical slope expressions, if any, predicted by these models, and, where indicated, some relationships between the models are discussed. In addition to the question of the electron energy spread, assumptions as to the number of energy states available to the molecule-ion were also considered. Situations discussed include: a single state and monoenergetic electrons, a continuum of states and monoenergetic electrons, a single state and an electron energy spread, a continuum of states and an electron energy spread, and fragmentation. Expressions were derived for the total ionization as a function of electron energy for four different models. (M.C.G.)