Electroosmosis is a fluid flow that is generated during electrophoresis in capillaries and microfluidic devices (e.g. Lab-on-a-Chip devices). Reproducible, stable electroosmosis is critical for developing microfluidic applications ranging from bioanalytical methods to chemical synthesis. Almost any change to the microchannel surface or the solution filling the channel will lead to changes in electroosmosis. These changes are difficult to study experimentally due to a lack of quantitative methods for measuring electroosmosis in channels with micrometer dimensions. We have developed an optical method for quantitative studies of electroosmosis dynamics based on the periodic photobleaching and detection of a dilute, neutral fluorophore added to the solution in the microchannel. We are using this technique to explore electroosmosis dynamics in simple microchannel systems filled with discontinuous solutions, a common experimental situation encountered in chemical applications of microfluidic devices.