A microfluidic device is used to measure electrode response to rapid changes in electrolyte composition. These changes in composition are achieved by switching the electrolyte flowing through an electrolyte channel containing a working and counter electrode. In the present investigation, the time response of the device was characterized by reducing tri-iodide ions at a rate controlled by mass-transfer to the electrode surface. Results are compared with finite-element simulations that assume an ideal two-dimensional parabolic flow. Simulated, theoretical, and experimental steady-state limiting current densities are all in excellent agreement. Experimental time constants are also in accord with simulations. (c) 2006 The Electrochemical Society.