An apparatus and a method for determining and/or monitoring at least one process variable of a medium in a container, comprising: a mechanically oscillatable unit, a driving/receiving unit for exciting the mechanically oscillatable unit to execute mechanical oscillations by means of an electrical exciting signal and for receiving and transducing mechanical oscillations into an electrical, received signal, an electronics unit, which electronics unit is embodied, to produce the exciting signal starting from the received signal, to set a predeterminable phase shift (Δϕ) between the exciting signal and the received signal, and from the received signal, to determine and/or to monitor the at least one process variable. A phase correction unit is provided, which phase correction unit is at least embodied, to ascertain a phase correction value (Δϕkor) from at least one process parameter dependent, characteristic variable of at least one component of the apparatus, especially the driving/receiving unit, and to set the predeterminable phase shift (Δϕ) in accordance with the phase correction value (Δϕkor).