It is shown that measurements of the chemical potential variation in a magnetic field could give valuable information about the magnetisation of thin films. Oscillations of μ in YBCO single crystal films evaporated on cleaved MgO substrates were observed. These oscillators may be ascribed either to the de Haas-van Alphen effect or to mesoscopic effects arising due to weak link currents. Large changes of μ resulting from superconducting shielding currents were observed in films evaporated on polished substrates. For films deposited on polished NdGaO 3 substrate the change of μ was mainly reversible. For films deposited on polished MgO, SrTiO 3 and LaAlO 3 substrates μ changed irreversibly.