The addition of water during Fischer-Tropsch synthesis over a 12.4 wt % CO/SiO2 catalyst led to a significant increase in CO conversion. A positive reversible impact of water on catalyst activity was observed at partial pressures of water of < 25 vol % added H2O. The CO conversion recovered to the reference activity after the termination of water addition. Therefore, this enhanced reversible activity for CO/SiO2 catalysts seems to be due to a kinetic effect of water. The catalyst was characterized using different techniques, including temperature-programmed reduction (TPR), Brunauer-Emmett-Teller (BET) surface area analysis, and H-2 chemisorption. The rate expression has been obtained over the catalyst, using a 1-L continuously stirred tank reactor (CSTR) that was operated at a temperature of 483 K, a pressure of 2.21 MPa (21.42 atm), H-2/CO feed ratios of 1.0-2.4, and 10%-70% CO conversion over a range of reactant partial pressures. 025 The data of this study are fitted by a power-law expression of the form r(CO) = kP(CO)(-0.25)PH(2)(0.5)/(1 + mPH(2)O/P-H2)1 where k = 0.89 mol h(-1) g(cat)(-1) and m = -0.155.