Abstract MoS 2 is a promising sulfur‐resistant candidate for water‐gas shift (WGS) reaction, and its catalytic efficiency can be promoted by alkali metal. In this work, a series of K‐modified MoS 2 /Al 2 O 3 catalysts with variable K/Mo ratios were prepared to elucidate the promotion role of potassium through IR spectroscopy studies. CO adsorption followed by IR spectroscopy shows that after potassium addition the number of accessible edge sites of MoS 2 slabs is decreased. Meanwhile, the downward shift of v (CO/MoS 2 ) indicates that the presence of potassium increases the electronic density of Mo atoms. This electronic effect activates terminal S atoms, favoring their reaction with CO to form COS at lower temperature than in absence of K. Additionally, conversely to the case of MoS 2 /Al 2 O 3 catalysts, little or even no formate species are observed on K‐modified MoS 2 catalysts during IR operando studies, implying a change of the WGS reaction route that would explain the beneficial effect of K on MoS 2 edge site activity.