The influence of fluorination on series of superconducting oxides YBa2Cu3O7−δ and Bi4(Ca, Sr)6Cu4O16+δ has been investigated. Fluorine gas treatments carried out at low temperature (20°C⩽TF2⩽100°C) yield the formation of a thin film of (oxy) fluoride (or oxyfluoride) on the surface of the material. This layer has been characterized by X-ray diffraction, electron microprobe analysis and Auger spectroscopy. In the case of YBa2Cu3O7−δ, the experimental conditions to achieve an efficient protection of the ceramics against hydrolysis and gas exchange have been determined. The influence of the fluorination treatments on the superconducting properties has been established from ESR, magnetic and conductivity measurements, in particular the improvement of the shielding effect-Meissner effect cycle.