The migration behavior and optimum separation of a set of substituted phenols is presented. A mathematical model that describes mobility in CZE in terms of fundamental constants of each solute (acid dissociation constant Ka, mobility of the dissociated acid microA-) and the pH of the buffer was used to predict the mobility of each solute as a function of pH. The resolution between each peak pair was calculated, and a window diagram of minimum resolution as a function of pH was employed in determining the optimum pH. A comparison of the predicted and actual electropherograms shows the usefulness of this technique.