We have compared the quality of Ba2YCu3O7−δ (BYCO) films grown by the BaF2 process on (100) substrates of the perovskites LaAlO3, LaGaO3, NdAlO3, NdGaO3, LiBaF3, and SrTiO3. The films were grown by coevaporation of Y, Cu, and BaF2 followed by a two-stage anneal. The high temperature stage of the anneal, the part of the process during which the BYCO crystal structure and morphology develop, has been varied. LaAlO3 and SrTiO3 support much better films, both electrically and structurally, than the other substrates. Of the oxides, NdGaO3 supports the worst films, while films on LiBaF3 are nonconducting. These results emphasize the overriding importance of chemical compatibility in determining the suitability of a potential substrate material. Unless this criterion is satisfied, the issue of lattice matching is unimportant.