Hydrolysis of waste poly(ethylene terphthalate) (PET) using solid acid catalyst in SCCO2 is presented in this work for the first time. The mechanism of PET chains scission was proved to be a combination of chain end and random chain scission by Fourier transform - infrared spectroscopy (FT-IR) and titration analysis. A new reaction kinetics model of PET hydrolysis in SCCO2 was setup by introducing the Arrhenius equation into an ordinary reaction rate equation, the frequency factor and apparent activation energy were expressed in terms of temperature and CO2 pressure, respectively. With this reaction kinetics model, the effects of temperature, and pressure were investigated. An interesting mechanism was proposed to describe the reaction process that both water molecules and hydroniums were carried and penetrated into the amorphous regions of the swollen PET by SCCO2, subsequently hydrolysis reaction preferentially took place in the amorphous regions of both surface and bulk of PET matrix. (c) 2014 American Institute of Chemical Engineers AIChE J, 61: 200-214, 2015