We have studied the effect of varying the postdeposition anneal conditions on the properties of thin films of YBa2Cu3O7prepared by coevaporation from Y, Cu, and BaF2. The temperature of this processing step determines both the microstructure of the films and the constituent phases. At low temperatures, BaF2is incompletely reacted and the predominant superconducting phase is Y2Ba4Cu8O20−x. At higher temperatures, the equilibrium distribution of phases, which includes YBa2Cu3O7, is formed. As the temperature is raised, the texture of the films also changes. The thermodynamics of the formation reaction and of the reaction between YBa2Cu3O7and various substrate materials is discussed.