Fluid flow in capillary microchannels is used in numerous applications in biotechnology (such as protein separation, fast DNA analysis, drug deliveries systems and viral filtration), in solid-state devices, and in catalytic devices. The current work presents the experimental validation for the electrokinetic theory in microchannels. Retardation of polar liquids, including de-ionized water, ethanol and propyl alcohol, is studied in microfabricated channels of several diameters. It was found that polar liquids flow about 6 percent more slowly than predicted by the classical hydrodynamic theory in microchannels, with the hydraulic diameter equal to 90 microns. For small microchannels with a hydraulic diameter of several microns, observed retardation is on the order of 70 percent. Collected experimental data have good correspondence with the electrokinetic model presented. Electrokinetic retardation of polar liquids in microchannels is based on the charge separation principle. Electrical charges are separated at the interface (near the channel wall). When liquid is forced downstream, it causes charge accumulation at one end of the microchannel. The streaming potential produced causes an upstream current that creates upstream counterflow. The resultant fluid flow is less than it would be for non-polar liquids. The higher the zeta-potential at the microchannel wall and the smaller the channel, the larger the resulting retardation. Modifications for the friction factor, as applied to microfluidics, are suggested. Recommendations to improve fluid flow in microchannels are made.