© 2017 Author(s). A piezoelectrically transduced micromechanical beam acoustic resonator is utilized to detect solid to liquid phase transitions in sub-nanoliter volume of Polyethylene Glycol (PEG) 1000. A lower frequency flexural mode and a higher frequency length-extensional mode are simultaneously monitored for changes in the device response. The phase transition of the PEG drop perturbs the acoustic loading of the resonator, bringing about characteristic and repeatable changes in the frequency response. The phase transitions in the drop are verified optically, and the resonant parameters are compared with the control case when the resonator is pristine without the presence of any PEG drop.