The protonation of an aqueous solution of two ampholytes AH and BH next to a gel buffered by immobilized acid moieties IH has been studied by finite element simulation in an iterative scheme. A ten species model has been formulated, taking into account transient diffusion and equilibrium kinetics of the two amphoteric species AH and BH, of water and of the immobilized species IN. This model has been developed to illustrate the pH evolution between an ampholyte solution and an Immobiline gel, and to study the influence of the Immobiline concentration on protons and ampholyte distributions. It has been demonstrated that a minimum initial Immobiline concentration of 10(-2) M is necessary to maintain the pH in the gel in contact with a closed chamber, when the difference between the isoelectric points of AH and BH is 4 and when the initial concentration of the ampholytes in solution is in the micromolar range. This approach provides a first theoretical framework for the recently developed Off-Gel(TM) electrophoresis.