Powdered samples of the high-temperature superconductors A Ba2Cu3O7−δ (A = Gd,Y) were treated with fluorine gas (100 Torr) at room temperature and 400 °C for varying times (12–64 h). Magnetic shielding measurements on fluorinated products showed that the superconducting volume fraction in treated samples was greatly reduced or even completely eradicated. All samples were structurally characterized by x-ray powder diffraction. Two yttrium samples, one fluorinated at 25 °C and one at 400 °, were also examined by neutron powder diffraction. For samples treated at room temperature, no change in the structure or composition of the products was apparent by either technique. However, samples fluorinated at 400 °C are tetragonal, with a = 3.8641 (3), c = 11.704(1) Å, and bulk composition corresponding to the formula YBa2Cu3F3.5O4.5. Nuclear activation analysis, nuclear reaction analysis, and Auger spectroscopy were used to determine fluorine concentration and distribution in the fluorinated materials. For samples treated at room temperature, fluorine was found primarily within approximately 1 μm of the surface of the product particles. No evidence for a fluorinecontaining superconducting phase was found in any sample; fluorine was found to be detrimental to superconductivity in all cases. These results suggest that the 123 oxides are sensitive to surface effects.