The reaction mechanism of molybdenum-based shift catalysts, which are sulfur-tolerant water-gas shift catalysts (sour-shift catalysts), were examined by elementary-reaction analysis. The elementary-reaction progression rate for a nickel/molybdenum/titanium (Ni/Mo/Ti) compound catalyst (as a low-temperature reactive catalyst) and cobalt/molybdenum/aluminum (Co/Mo/Al) compound catalyst (as a conventional catalyst) were investigated. The results of this investigation show that the elementary-reaction progression rate of the Ni/Mo/Ti catalyst is higher than that of the Co/Mo/Al catalyst at 200 degrees C (which is representative of the low -temperature region). This tendency accorded with that of shift activity. This result suggests that the shift activity depends on improved dispersibility of active sites by the support effect of TiO2 and the strong electron-withdrawing effect of the nickel promoter. Moreover, separately from the reaction mechanism of the sour-shift catalyst proposed up to now, a new mechanism, by which H2S in reaction gas contributes to the progress of the shift reaction in the elementary reaction process, is proposed, and as a result of elementary-reaction analysis, we confirmed this new mechanism must be progressed. (C) 2016 Elsevier B.V. All rights reserved.