In crystalline thin film growth a prerequisite is substrate surfaces with a stable and uniform structure and chemical composition. Various substrate treatments were used to obtain atomically smooth, step-and-terrace (111)-oriented SrTiO3 with uniform cation layers at the surface, i.e., single termination. The surface control enables subsequent layer-by-layer epitaxial growth of perovskite thin films of La0.7Sr0.3MnO3, LaFeO3, and BaTiO3. Reflection high-energy electron diffraction and electron energy loss spectroscopy revealed that a single chemically intermixed (A,A′)BO3 perovskite layer formed at the interface. As the terminating layer of (111) SrTiO3 is polar, a surface reconstruction consisting of TiOx surface layers is expected, and the intermixing at the interface can be understood as A′-cations from the film material compensating an A-cation deficient substrate surface during initial growth. This finding has important consequences for engineered interfaces between perovskite thin films and polar substrate facets.