Unusually high efficiencies (up to 2.5 million plates m -1 ) during the capillary electrochromatographic analysis of partially ionized anionic-neutral compounds have been observed under reversed-phase conditions using a standard C 18 stationary phase. An explanation has been proposed in terms of nonequilibrium conditions caused by pulses of stronger or weaker solvent that arise from the sample. The increased efficiencies are observed when the migration time of the analyte is closely matched to the elution time of sample-induced discontinuities in the mobile phase. Repeatability and hence the feasibility to control the system have been demonstrated along with the effect that separation parameters such as mobile-phase organic solvent content, ionic strength, and separation voltage have on peak efficiencies, areas, heights, and asymmetry. Van Deemter plots show that the B term (axial molecular diffusion) is the only major contributor to peak dispersion. A reduced plate height of 0.1 was obtained. The implications of this phenomenon and its ability to cause concurrent electrophoretic effects during the analysis of charged species, thus leading to even greater efficiencies, are discussed.