New measurements of the electrophoretic mobility of T-cell model systems have been carried out and analyzed to obtain the dynamic variation in mobility in small titration increments during separate upscale and downscale sweeps in pH. We demonstrate that a plot of p lambda vs p[NaCl] has been found essential in evaluating the consistency of electrophoretic mobility measurements at different (1:1) electrolyte concentrations and show, for the first time, that electrophoretic mobility measurements as a function of pH can reflect different rates of the respective ionization and association that occur in the surface functional groups as a consequence of the different changes in the hydration-dehydration reactions involved. Differences found between the upscale and downscale sweeps suggest that it is easier to protonate a protein cell surface than to deprotonate it. The effect is most pronounced at the highest salt concentration (similar to that which exists for the cells in their native state) and becomes less pronounced as the salt concentration is lowered. The effect is interpreted as a result of the different changes in the state of hydration as a proton moves from the bulk through the double layer to a surface group and the reverse. The effect occurs with both replicating and activated T-cells. This latter result may be of biological significance and particularly relevant to HIV-1 infection, since during male-to-female transmission, the environment where most infections occur supports this protonation effect.