The additives that are frequently used in chromatography to modulate retention of feed components can affect not only the thermodynamics but also the dynamics of the separation process. These mobile phase additives, or modulators, can themselves adsorb significantly and thus undergo spatial variations in concentration due to the competition among all the adsorbates found in the column. In particular, displacement chromatography in the absence of feed components generates a band of enriched modulator ahead of the displacer front. Analytical expressions for the concentration of the modulator-enriched zone are found for Langmuirian and multivalent ion-exchange isotherms. The significance of these results to displacement separations of feed mixtures is that the extent of enrichment is necessary in determining whether a displacement train of the feed components can be formed in the presence of the modulator. An explicit condition for the formation of a displacement train, termed "the generalized Gluckauf condition," is presented. A representative example is given to show that a feed component that does not obey the generalized Gluckauf condition moves too fast to be caught by the displacer front and therefore elutes. An analog of this generalized condition is shown to be applicable even when complete multicomponent isotherm data for all the feed components, as well as the displacer and modulator, is not available. It may sometimes be useful to prevent or minimize the effect of the modulator on the separation. To this end a hodograph analysis is used to provide an explicit bound on the critical displacer concentration below which modulator effects can be considerably reduced.