The pH transients that occur during isoelectric trapping separations as a result of the removal of nonampholytic ionic components have been re-examined. Salts containing strong electrolyte anions and cations, both with equal and dissimilar mobilities, have been studied using anodic and cathodic buffering membranes whose pH values were both equidistant and nonequidistant from pH 7. The direction and magnitude of the pH transient (acidic or basic) was found to depend on both the mobilities of the anion and cation (mu(anion)/mu(cation)) and the pH difference between pH 7 and the pH of the buffering membranes (vertical bar pH(anodic)(memb) - 7 vertical bar/vertical bar 7 -pH(cathodic)(memb)vertical bar). When vertical bar pH(anodic)(memb) - 7 vertical bar/vertical bar 7 - pH(cathodic)(memb)vertical bar = 1, mu(anion)/mu(cation) < 1 leads to an acidic pH transient, mu(anion)/mu(cation) = 1 eliminates the pH transient and mu(anion)/mu(cation)> 1 leads to a basic pH transient. When mu(anion)/mu(cation) 1, vertical bar pH(anodic)(memb) - 7 vertical bar/vertical bar 7 - pH(cathodic)(memb)vertical bar < 1 leads to a basic pH transient, vertical bar pH(anodic)(memb) 7 vertical bar/vertical bar 7 - pH(cathodic)(memb)vertical bar = 1 eliminates the pH transient and vertical bar pH(canodic)(memb) - 7 vertical bar/vertical bar 7 - pH(cathodic)(memb)vertical bar > 1 leads to an acidic pH transient. By selecting appropriate anodic and cathodic buffering membranes to adjust the vertical bar pH(anodic)(memb) - 7 vertical bar/vertical bar 7 - pH(cathodic)(memb)vertical bar value, pH transients caused by dissimilar anion and cation mobilities can be avoided.