The escalating demand for sustainable and safe cosmetic products has catalyzed the adoption of bioplastic substrates, particularly those based on starch, in the development of facial sheet masks. This chapter provides a comprehensive review of the use of Fourier Transform Infrared (FTIR) spectroscopy for the structural and compositional characterization of starch-based bioplastics within cosmetic applications. FTIR is highlighted as a key analytical method, capable of monitoring functional group changes, molecular interactions, and the impact of additives such as plasticizers and fillers on film performance. Multiple case studies demonstrate how FTIR distinguishes between native and modified starches, elucidates effects on moisture resistance, flexibility, and bioactive incorporation. These insights reinforce FTIR as an indispensable quality control and material development tool for next-generation cosmetics, ultimately supporting the industry’s environmental and safety objectives.