Phase transformations of styrene (C8H8) at room temperature and up to 32 GPa have been studied by visual observations, Fourier transform infrared spectroscopy, and Raman scattering. At room temperature, styrene freezes at 0.3 GPa and undergoes a solid-to-solid transformation around 3.5 GPa. Around 15 GPa the extent of the reaction monostyrene-->polystyrene becomes significant. At 32 GPa the reaction is nearly completed and, contrary to the general trend, the phenyl group is still stable at this pressure. This uncommon behavior is very likely correlated with the amorphous character of the polystyrene formed under pressure.