In this article, we determined the composition of phenolphthalein by IR spectroscopy using a Shimadzu FTIR spectrometer. The wavenumber range is between 3300 and 500cm^-1. Phenolphthalein contains the following functional groups: O-H, C=O, C=C, C-C and aromatic compounds. Phenolphthalein is a widely used organic compound, notably as an acid–base indicator, whose chemical composition and functional groups can be effectively characterized by infrared (IR) spectroscopy. In this study, IR spectroscopic analysis was employed to determine and confirm the molecular structure of phenolphthalein. The IR spectrum exhibited characteristic absorption bands corresponding to its functional groups, including a broad band in the region of 3200–3600 cm⁻¹ attributed to phenolic O–H stretching vibrations, strong absorptions near 1700–1750 cm⁻¹ associated with the carbonyl (C=O) group of the lactone ring, and bands in the 1500–1600 cm⁻¹ region corresponding to aromatic C=C stretching vibrations. Additional peaks observed between 1000–1300 cm⁻¹ were assigned to C–O stretching vibrations. These spectral features are consistent with the known molecular formula (C₂₀H₁₄O₄) and structure of phenolphthalein. The results demonstrate that IR spectroscopy is a reliable and effective technique for identifying the chemical composition and confirming the functional groups present in phenolphthalein.