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In this work, the operational mechanism of single-layer light-emitting electrochemical cells (LECs) based on the small molecule tris(2,2’ bipyridyl) ruthenium(II) [Ru(II)] was investigated using capacitance and resistance measurements. The current–voltage and capacitance–voltage characteristics of such devices suggest that an electrochemical junction is formed during operation with a high electric field across the junction. A similar mechanism has been proposed for polymer LECs. In the case of Ru(II) devices, electrically conducting regions adjacent to the electrodes are the result of mixed-valent states that form due to oxidation and reduction of the complex. The junction thickness is a function of the type of counterions used and the operating voltage. Thinner junctions were observed for devices with high ionic conductivity and at higher operating voltages. Transient capacitance and resistance measurements show that the junction formation is faster in devices with higher ion mobility and during higher operating voltages. In addition, the capacitance and resistance exhibit a relaxation time after the device is turned off. This relaxation shows that the electrochemical junction stays present in a device for some time (several seconds to minutes) once a device is turned off. The electrochemical junction disappears as the counterions relax back. Furthermore, a theoretical model is presented, which shows that due to the concentration gradient of mixed-valent states during operation, an electric field has to be present in the device. The model also shows that there can be no local charge neutrality in the bulk of the device during operation. © 2003 American Institute of Physics.