A generalized model that describes apparently "non-Nernstian" equilibrium responses of ionophore-based ion-selective electrodes (ISEs) is presented. It is formulated for primary and secondary ions of any charges that enter the membrane phase and independently form complexes with the ionophore, respectively. Equations for the phase boundary potential model were solved numerically to obtain whole response curves as a function of the sample activity of the primary ion, and analytical solutions could be obtained for apparently non-Nernstian response sections in these response curves. Ionophore-based ISEs can give three types of apparently non-Nernstian equilibrium responses, i.e., apparently "super-Nernstian", "inverted-Nernstian", and "sub-Nernstian" responses. The values of the response slopes depend on the charge numbers of the primary and secondary ions and on the stoichiometries of their complexes with the ionophore. The theoretical predictions for super-Nernstian responses agree well with the experimental results obtained with ISEs based on acidic ionophores or metalloporphyrin ionophores. Also, theoretical response curves with inverted-Nernstian slopes were found to be similar in character to the pH responses of Ca2+-selective electrodes based on organophosphate ionophores, which have been known to exhibit a so-called "potential dip". The quantitative understanding of apparently non-Nernstian response slopes presented here provides an insight into ionophore-analyte complexation processes in ISE membranes and should be helpful for the design of new ionophores.