The measurement of the composition of ethanol-CO2 mixtures at high pressures is important in many applications involving supercritical fluids such as drying of alcogels or release of MEMs. Resonant frequency and quality factor (Q-factor) of microcantilevers immersed in ethanol-CO2 mixtures were measured at a temperature of 308 K and pressure range from 8 MPa to 22 MPa. The measurements were carried out for different mixture compositions ranging from 0.91 to 6.16 wt% of ethanol in CO2. At a given pressure and temperature, the resonant frequencies were found to decrease linearly with the increasing ethanol weight percent in the mixture. The sensitivity of the resonant frequency to changes in composition was found to increase with decreasing pressure. The experimental results show that ethanol-CO2 mixture composition can be determined with good accuracy using mainly the measured resonant frequency of microcantilevers.