The popularity of ethanol as an alternate energy source to partially replace gasoline has been growing. However, an internal combustion engine needs different air/fuel ratio setting depending upon the ethanol/gasoline blend ratio, because the energy content of ethanol and gasoline is different. Therefore, an on-board ethanol concentration sensor is crucial, because it would make the ethanol/gasoline blend ratio available to the engine control unit. In this work, an ethanol concentration sensor and sensing technique are developed. This approach consists of measuring both the impedance modulus and phase angle of an ethanol/gasoline blend, and then estimating the capacitance and the ethanol concentration from these two parameters. Further, a lower cost, but equally effective technique that uses only the impedance modulus to estimate the ethanol concentration in a fuel blend is also demonstrated, and an on-board ethanol concentration sensor is developed using this method to monitor the fuel blend in real time. This sensor can be installed inside a Fuel Module located in a gas tank; or installed in a fuel line. The advantages and disadvantages of these two packaging designs are also discussed.