Thin films of Cu, Y and BaF2 are co-condensed in ultrahigh vacuum onto SrTiO3 (1 0 0)-substrates at room temperature without any additional oxygen supply. It is found that the temperature of the ex situ fluorine-oxygen exchange reaction in flowing wet oxygen essentially determines whether the Y1Ba2Cu3O7 - x-phase (Tex = 850°C) or the Y2Ba4Cu8O16 - x-phase (Tex = 800°C) forms or any mixture of both phases in between. Small-angle X-ray diffraction verifies the strictly epitaxial growth of the superconducting phases. The variation of the composition around the ideal 1,2,3-stoichiometry affects only the superconducting and normal conducting properties which are measured using the four-probe Montgomery method. The values of the normal state resistivity and its temperature dependence are discussed in combination with a reduced cross section of the conduction paths in the films. © 1990.