Palladium-silver alloy films were prepared by slow simultaneous evaporation in vacuum from separate sources onto the inner surface of a glass reaction vessel held at 400 °C; with attention to source geometry an even composition was obtained. Electron micrographs showed extensive coalescence of the crystallites in silver-rich films changing to a compact mass of small crystallites at high palladium contents. Alloy films which were not used as catalysts showed good bulk homogeneity (symmetrical X-ray diffraction profiles, lattice constants in agreement with bulk values, etc.). After use as catalysts for ethylene oxidation, silver-rich films were apparently still homogeneous but palladium-rich films reproduced the phenomena of Pd-Ag-H alloys (expanded lattices and double diffraction peaks in some cases, ascribed to α and β phases simultaneously present). The extent of the positive deviation in lattice constant from expected values correlated with the utilization of oxygen from the reaction mixture. However, pure palladium film catalysts always gave the bulk lattice constant in contrast to alloy films with as little as 1% silver which were apparently well charged with hydrogen after reaction. The ability of oxygen alone at reaction temperature, 250 °C, to cause some separation of the alloy film towards its component metals was also demonstrated and discussed in relation to results using an alloy foil.