This paper studied the effects of the additives including Zr, Mg and K on the catalytic behaviors of Co/Al₂O₃ during the steam reforming of acetic acid. It was found that Zr additive had negligible effect on the performance of the cobalt catalyst, while Mg additive could markedly lower the activity and selectivity of the catalyst. In comparison, K showed remarkable positive effects on the cobalt catalyst. The additives affect the physiochemical properties of the catalyst in varied ways, imposing distinct effects on the performance of the catalysts. Zr additive did not affect reduction behaviors of cobalt oxides or distribution of metallic cobalt sites, thus the effects of Zr additive on performances of the catalyst was insignificant. Mg additive, in converse, led to reduction temperature of cobalt oxides shifting upwards, which negatively affected the formation of metallic cobalt sites on surface of the catalyst and consequently on the catalytic activity and stability. K additive could drastically promote the reduction of cobalt oxides, significantly promoting the catalytic activity and stability. Further to this, K additive could also effectively reduce methane formation during steam reforming of acetic acid through suppressing the methanation reactions. In addition, K additive could enhance the resistance of metallic cobalt towards oxidation, enhancing the catalytic stability, while Zr or Mg additive did not have such an effect.