Apparatus for determining and/or monitoring at least one process variable of a medium, particularly a density, a viscosity or a level of the medium in a vessel, with - a structure capable of vibrating mechanically (5, 27), which has at least one vibration property that depends on the process variable, - an electromechanical transducer (9, 25) which has at least one piezoelectric element (7, 7a, 7b) -- which excites the structure (5,27) to execute mechanical vibrations by means of an excitation signal supplied to the transducer (9, 25), and -- which converts the resulting vibrations of the structure (5, 27) into a reception signal which corresponds to a superposition of the excitation signal and a wanted signal that reflects the vibrations, - a reference element parallel to the transducer (9, 25) that is exposed to the excitation signal, via said element a reference signal (S R) is measured, said reference signal being independent of the vibration and corresponding to the excitation signal, and - electronics, -- which extract the wanted signal by means of the reception signal and the reference signal (S R), and -- which determine and/or monitor the process variable by means of the wanted signal, characterized in that the electronics comprise a temperature measurement unit, which is designed in such a way that in the temperature measuring mode it exposes the transducer (9, 25) and the reference element to an auxiliary signal whose frequency (f H) is outside a frequency range specified by a resonance frequency (f r) of the vibration structure (5, 27), and determines a temperature of the transducer (9, 25) using the wanted signal extracted in the temperature measuring mode. - a structure capable of vibrating mechanically (5, 27), which has at least one vibration property that depends on the process variable, - an electromechanical transducer (9, 25) which has at least one piezoelectric element (7, 7a, 7b) -- which excites the structure (5,27) to execute mechanical vibrations by means of an excitation signal supplied to the transducer (9, 25), and -- which converts the resulting vibrations of the structure (5, 27) into a reception signal which corresponds to a superposition of the excitation signal and a wanted signal that reflects the vibrations, - a reference element parallel to the transducer (9, 25) that is exposed to the excitation signal, via said element a reference signal (S R) is measured, said reference signal being independent of the vibration and corresponding to the excitation signal, and - electronics, -- which extract the wanted signal by means of the reception signal and the reference signal (S R), and -- which determine and/or monitor the process variable by means of the wanted signal, characterized in that the electronics comprise a temperature measurement unit, which is designed in such a way that in the temperature measuring mode it exposes the transducer (9, 25) and the reference element to an auxiliary signal whose frequency (f H) is outside a frequency range specified by a resonance frequency (f r) of the vibration structure (5, 27), and determines a temperature of the transducer (9, 25) using the wanted signal extracted in the temperature measuring mode. -- which excites the structure (5,27) to execute mechanical vibrations by means of an excitation signal supplied to the transducer (9, 25), and -- which converts the resulting vibrations of the structure (5, 27) into a reception signal which corresponds to a superposition of the excitation signal and a wanted signal that reflects the vibrations, - a reference element parallel to the transducer (9, 25) that is exposed to the excitation signal, via said element a reference signal (S R) is measured, said reference signal being independent of the vibration and corresponding to the excitation signal, and - electronics, -- which extract the wanted signal by means of the reception signal and the reference signal (S R), and -- which determine and/or monitor the process variable by means of the wanted signal,