A generalized quasi-thermodynamic theory of the solvent polymeric ion-selective membranes containing ion-exchanging sites (charged carrier) and a co-exchanger (ionic additives) and in contact with two mixed aqueous solutions of electrolytes IX₂ and JX is developed. To account for diffusion potential it is proposed to assume various profiles of species in the diffusion layer. A computer simulation of the potentiometric signal and of its selectivity is fulfilled. A surprising result of the simulations is that though the value of diffusion potential achieves sometimes several tens of mV, it is almost the same for different profiles assumed. If being profile-independent, the diffusion potential occurs also time-independent. Therefore the usually fast response of ISEs actually does not mean that only boundary potentials matter. The variability of selectivity coefficients to a large extent is also caused by diffusion potential. Artificial elimination of the diffusion potential with ionic additives to membranes based on calcium dioctylphenylphosphate allows one to eliminate the peculiarities like Ca/pH potential dips and also to stabilize selectivity coefficients.