Thermally evaporated thin films of TlI have been found to grow epitaxially on KCl(100) surfaces equally with two mutually perpendicular orientations. We present x-ray and optical data to show that the TlI(004) plane grows with its bo axis parallel to each of the two KCl 〈110〉 directions that lie on the (100) surface. This unusual double epitaxial microstructure with mutually perpendicular orientations results in optical scattering by the diffraction of light passing from one biaxially birefringent orientation to another. Due to the highly desirable properties of TlI as an optical coating material for high powered infrared laser components, several techniques have been devised to minimize the optical scattering and improve film homogeneity. Data are presented on the use of a thin buffer layer which severely reduces the epitaxial growth of TlI resulting in homogeneous films similar to those deposited on glass. Epitaxial TlI films may also be made with no visible light scattering by alternating thin layers of TlI with those of another material. This alternating layer method takes advantage of the polymorphism of TlI in thin layers to produce films in the cubic phase. We show by x-ray and optical measurements that each TlI layer possesses the CsCl structure at room temperature.