A new impedance-based sensor to monitor drug delivery from an implantable microelectromechanical systems (MEMS) device has been fabricated and tested. The sensor consists of two electrodes on opposing sides of a pyramidal drug reservoir. The dissolution of the drug and advance of solution into the reservoir cause the impedance to change over time. A 100 times scale model of the sensor was constructed to examine the effects of electrode geometry on solution resistance. An equivalent circuit was formulated to interpret the impedance signal in terms of the resistance and double-layer capacitance of the solution in the reservoir. The circuit was validated by impedance measurements on reservoirs filled with phosphate-buffered saline solutions of varying concentrations. The sensor was then used to monitor the dissolution of the model drug mannitol from the drug delivery MEMS device. The measured solution resistance and double-layer capacitance are related to the rate of transport of drug from the device, making this sensor a potential instrument for noninvasive monitoring of drug transport from the implant in vivo. Experimental results agree closely with the expected values of capacitance, resistance, and dissolution time calculated from physical parameters. (C) 2004 The Electrochemical Society.