Following a tertiary recycling option for plastics focused on the FCC process, styrene-based polymers were dissolved in benzene and injected over fresh and equilibrium samples of a commercial FCC catalyst. Reaction times were up to 12 s in a discontinuous fluidized bed reactor at 550 degrees C. The major reaction products were ethylbenzene, benzene, toluene, and styrene, and the production of coke was very significant, A reaction mechanism which includes polymer thermal cracking, surface oligomerization of styrene molecules, and further cracking, in parallel to hydrogen-transfer reactions, was delineated based on the comparison of the catalytic behaviors of the different samples. Increases in the yields of ethylbenzene, styrene, and coke are predictable for the industrial conditions.