A study of CO hydrogenation on La3+-promoted Co/SiO2 catalysts (La/Co atomic ratios of 0-0.75) has been performed using both global rate measurements and steady-state isotopic transient kinetic analysis (SSITKA). Aqueous impregnation (the means of adding La3+) of Co/SiO2 led to a significant decrease in the number of intermediates leading to methane during CO hydrogenation, but it did not alter the nature of the active sites. In a previous characterization paper (19) La3+ promotion of Co/SiO2 was shown to moderate the strong Go-support interactions brought about by aqueous impregnation of the Co/SiO2 and to enhance the reducibility of the Co oxide, La3+ addition to the Co/SiO2 catalyst was found to lead to a significant increase in the activity for Fischer-Tropsch synthesis (on a per gram Co basis). The SSITKA results showed that the concentration of active intermediates leading to methane increased with increase in La/Co ratio. However, the TOE the pseudo-first-order rate constant, the reactivity distribution function, and the activation energy were not affected by La3+ addition. Thus, it can be concluded that La3+ did not change the nature of the active sites,just their concentration, This increase in the concentration of active sites follows exactly the increase in surface Co-0. La3+ promotion (La/Co greater than or equal to 0.1) also caused the chain growth probability and the olefin/paraffin ratio to increase significantly at 220 degrees C and for H-2/CO = 2.