We focus on the application of infrared spectroscopic ellipsometry (IRSE) to simultaneously determine phase and microstructure of mixed-phase thin films such as polymorphic-polycrystalline boron nitride (BN) thin films deposited by magnetron sputtering on (100) silicon. We discuss a recently presented microstructure-dependent model for infrared optical properties of mixed-phase thin films which contain anisotropic materials. In particular, the IRSE data are sensitive to the pure or mixed hexagonal (h) and cubic (c) BN thin film layer structure, phase composition and average grain c-axis distribution of the hexagonal phase.