This study focused on the impacts of the varied extent of interaction between nickel and alumina in Ni/Al2O3 on the catalytic behaviors and reaction intermediates formed in methanation of CO2 and steam reforming of acetic acid. The calcination of alumina at high temperature eliminated some "reactive sites", alleviating the interaction between nickel species and alumina and significantly impacting the catalytic activities. The interaction between nickel and alumina was much stronger and the nickel dispersion was also higher over the 5% Ni/Al2O3-600 degrees C than over 5% Ni/Al2O3-1000 degrees C. However, the activities of the former catalyst was remarkably lower. The interaction between nickel and alumina played more profound influence than nickel dispersion on the catalytic activity. The weak interaction between nickel and alumina would help to "release" or "free" the nickel species for catalytic reactions, which, however, facilitated the migration and sintering of nickel. The in situ DRIFTS studies of methanation showed that the weaker interaction between alumina and nickel promoted the conversion of HCOO*, a key intermediate from methanation of CO2, to CH4. The properties of coke formed in steam reforming was also affected by the interaction between nickel and alumina. Fibrous form and amorphous form of coke tended to form over the catalyst with stronger interaction, while carbon nanotube tended to form over the catalyst with weaker interaction.