Results of the FT-IR studies have shown that the carbonaceous deposit formed over gamma-Al2O3 has carboxylate character, while that formed over silica-alumina and HX zeolite is a mixture of the above and conventional (aromatic) coke. The deposit formed over HZSM-5 zeolite is composed mainly of conventional coke, however, species containing acetyl groups were also present. No carbonaceous deposit forms over A zeolite.