Abstract The influence of barium addition to a Ni/Al2O3 catalyst on the reaction intermediates formed, the activity, resistance of the catalyst to coking, and properties of the coke formed after acetic acid steam reforming were investigated in this study. The results showed the drastic effects of barium addition on the physicochemical properties and performances of the catalyst. The solid-phase reaction between alumina and BaO formed BaAl2O4, which re-constructed the alumina structure, resulting in a decrease in the specific surface area and an increase in the resistance of metallic Ni to sintering. The addition of barium was also beneficial for enhancing the catalytic activity, resulting from the changed catalytic reaction network. The in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study of the acetic acid steam reforming indicated that barium could effectively suppress the accumulation of the reaction intermediates of carbonyl, formate, and C=C functional groups on the catalyst surface, attributed to its relatively high ability to cause the gasification of these species. In addition, coking was considerably more significant over the Ba–Ni/Al2O3 catalyst. Moreover, the Ba–Ni/Al2O3 catalyst was more stable than the Ni/Al2O3 catalyst, owing to the distinct forms of coke formed (carbon nanotube form over the Ba–Ni/Al2O3 catalyst, and the amorphous form over the Ni/Al2O3 catalyst).