Abstract The surfaces of c-axis oriented YBa 2Cu 3O 7−δ (YBCO) thin films prepared by DC-magnetron sputtering on SrTiO 3 (100) substrates were studied by means of X-ray photoelectron spectroscopy (XPS). The measurements were carried out subsequently during a period of a three-months-storage of the samples in ambient air as well as immediately after the film deposition without exposing the film surfaces to atmosphere. A chemical reaction with formation of hydroxides and carbonates on top of the film surface was observed in the case of the air-exposed samples. The thickness of this top layer was found to develop proportionally to the square root of time with a time constant of 0.1 nm/h 0.5. Accompanying measurements of the critical temperature ( T c) and the critical current density ( j c) show that T c and j c of the films are normally not affected by the top layer formation and remain nearly unchanged over periods longer than eight months.