Displacing one electrolyte solution with another solution in a microchannel is often required in many biomedical lab-on-a-chip devices. This paper discusses both theoretical and experimental studies of electroosmotic flow in a capillary with one electrolyte solution displacing another solution. A theoretical model was developed to predict the electroosmotic flow displacing process. This model considered the mixing process between the two different solutions and the induced pressure gradient in the capillary due to the different electrolyte solutions and hence the different electrokinetic conditions in different sections of the capillary. In the experiments, deionized ultrafiltered water, 10(-2) M KCI solution, 10(-4) M KCI solution, and 10(-4) M LaCl3 solution were used as the testing fluid. Polyamide-coated silica capillary tubes 100 it m in internal diameter and 10 cm in length were used in this study. The nonlinear change of the current with time was found during such a displacing process under a constant applied electrical field. A good agreement between the experimentally measured current change and the model prediction of the current change was found. The characteristics of the mixing process are also discussed in the paper.