An apparatus for determining and/or monitoring at least one physical or chemical, process variable of medium having at least one oscillatable unit, which produces, and/or receives, mechanical oscillations. Included is at least one tuning unit, whose stiffness is changeable and which is embodied in such a manner and connected in such a manner with the oscillatable unit, or is a part of the oscillatable unit in such a manner, that at least the resonance frequency of the oscillatable unit is changeable via the tuning unit. A corresponding method is also noted.