In this study, steam reforming of guaiacol over Ni/Al2O3 and Ni/SBA-15 catalysts were evaluated, aiming to investigate the impacts of the two supports with varied pore sizes, level of interaction with nickel species on behaviors of the catalyst and properties of the coke formed. The results indicated steam reforming of guaiacol involved a complex reaction network as isomerization and cracking of guaiacol produced many by-products like furans, cyclopentones and heavier aromatics. Ni/SBA-15 and Ni/Al2O3 catalyst only showed appreciable activities for guaiacol reforming at 600 degrees C. Although nickel size was bigger in Ni/SBA-15, Ni/SBA-15 showed superior activity to that of the Ni/Al2O3. In situ DRIFTS studies of guaiacol reforming indicated that the strong absorption of the phenolics on Ni/Al2O3 catalyst might lead to the coverage of the active sites and the low catalytic activity. Although coking was more serious over Ni/SBA-15, Ni/SBA-15 was much more stable than Ni/Al2O3. The TG-MS, Raman, FT-IR, elemental analysis, and TEM characterizations indicated the coke over Ni/Al2O3 catalyst was mainly aliphatic, thermally unstable and existed in an amorphous form, while the coke over Ni/SBA-15 catalyst was mainly aromatic, thermally stable and existed in the form of carbon nanotubes.