Effects of pyrolysis temperature and terephthalic acid (TPA) gas concentration on TPA degradation as a model substance for poly(ethylene terephthalate) (PET) degradation products were investigated with emphasis on benzene yield and decrease of carbonaceous residue. TPA was decarboxylated using a spiral tube reactor filled with CaO. The best results were achieved at 600 °C and TPA gas load of 28 mg L-1, yielding 84% benzene. Of the initial carbon, 29% remained in the reactor, consisting of carbonate and carbonaceous residue. Benzene purity was more than 97 wt % under all reaction conditions. Low TPA gas loads and high gas velocities resulted in good benzene yields and purities because TPA gas deeply penetrated the CaO fixed bed before adsorption. This resulted in lower TPA concentration at the CaO surface, reducing the interference between neighboring TPA molecules and limiting carbonaceous residue formation.