Abstract If zones do not contain any sample ionic species and contain only ionic species of the background electrolyte (BGE) with a composition, however, different from that of the BGE with regard to the concentrations of these ionic species and/or the pH in the zone, they can migrate with a specific mobility through the BGE. Of course, these BGE-like peaks will only be visible in electropherograms if the ionic species of the BGE can be observed by the detector. In this paper we propose to apply the term system peaks exclusively for BGE-like peaks and a model is given for the calculation of the mobilities of such system peaks. These system peaks can be created by directly introducing BGE-like zones with a composition different from the BGE or can be induced by the injection of a sample or even water. Several examples of system peaks are given.