We demonstrate the application of Spectroscopic Ellipsometry (SE) for identification of secondary phase MoSe2 in polycrystalline Cu2ZnSnSe4 (CZTSe) samples. A MoSe2 reference sample was analyzed, and its optical constants (ε1 and ε2) were extracted by SE analysis. This dataset was implemented into an optical model for analyzing SE data from a glass/Mo/CZTSe sample containing MoSe2 at the back side of the absorber. We present results on the n and k values of CZTSe and show the extraction of the thickness of the secondary phase MoSe2 layer. Raman spectroscopy and scanning electron microscopy were applied to confirm the SE results.