There are a number of benefits associated with electroosmotic flow including a plug flow profile (reduced transaxial contribution to zone broadening) and a mobile phase velocity that is independent of the plate length and particle size (assuming Jouleheating is not a limiting factor) when compared with pressure-driven flow. Early investigations in open-layer systems showed poor reproducibility of migration distances [40,41]. This arose from the evaporation of the mobile phase from the layer due to Joule heating. Under conditions of excessive wetting or dry-ing of the layer, degradation of the separation quality occurs.Most recent studies have employed an over pressured separation chamber to eliminate the vapor phase above the layer affordingim proved separation quality. A buffered mobile phase is required for pressurized planar electrochromatography. Most separations so far have utilized chemically bonded layers in the reversed-phase mode. The mobile phase is driven through the prewet layer by a high voltage established by two electrodes in independent buffer reservoirs connected to opposite sides of the layer. Plate heights of about 12–15 m have been obtained for HPTLC layers in a shorter time and with longer migration distances than possible with capillary-controlled flow development [42].