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Microelectromechanical systems (MEMS)-based resonant mass sensors have been extensively studied due to their high sensitivity and small size, making them very suitable for detecting micro- or nanosized particles, as well as monitoring microscaled physical processes. In a range of physical and biological applications, accurate estimation and precise control of the evaporation process of microdroplets are very important. However, due to the lack of appropriate measurement tools, the evaporation process of microdroplets has not been well characterized. Here, we introduce a self-oscillating MEMS mass sensor with a uniform mass sensitivity to directly measure the mass changes of evaporating microdroplets. The mass sensor has a unique spring structure to provide spatially uniform mass sensitivity. The sensor's velocity is fed back to the actuation signal to induce self-oscillation, enabling rapid determination of the resonant frequency. The evaporation rates of single microdroplets of dimethyl sulfoxide and water at various temperatures are obtained. With the measured evaporation rates and the simulated surface area of the microdroplet, the enthalpies of vaporization of both liquids are extracted and found to be in agreement with those in the literature. The method developed in this work can be a valuable tool to enhance our understanding of microscaled physical processes involving rapid mass change, such as evaporation, deposition, self-assembly, cryopreservation, and other biological applications.