Investigating effects of thymolphthalein on thermo-oxidative stability of polypropylene in melt and solid states A two-step procedure was adopted for synthesis of thymolphthalein stearate, i. e. stearic acid was firstly converted to stearoyl chloride in the presence of thionyl chloride and then stearoyl chloride rendered thymolphthalein stearate by reacting with thymolphthalein. Differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy confirmed the formation of thymolphthalein stearate. Thymolphthalein stearate was then incorporated into polypropylene through solvent premixing followed by compression molding. Effects of the additive on thermo-oxidative stability of polypropylene were investigated using oxidation onset temperature (OOT) and oxidative induction time (OIT) measurements and Fourier transform infrared (FTIR) spectroscopy. It was shown that thymolphthalein stearate has a remarkable ability in modifying the thermo-oxidative stability of the polymer in both melt and solid states. Finally, the obtained results were interpreted in terms of the radical scavenging ability of thymolphthalein moieties of the additive.