The kinetics of water gas shift (WGS) reaction over an Fe-Mn catalyst under Fischer-Tropsch synthesis (FTS) reaction conditions is studied in a spinning basket reactor. Experimental conditions are varied as follows: temperature of 533-573 K, reactor pressure of 10.0-26.5 bar, H-2/CO feed ratio of 0.66-2.0 and space velocity of 0.66-2.65 X 10(-3) Nm(3) kg(cat)(-1) s(-1). By separately fitting WGS kinetics parameters with experimental data, which is possible in the spinning basket reactor with neglecting concentration and temperature gradients, different kinetics models of WGS are derived and discriminated on the basis of four sets of WGS elementary reactions. Kinetics experimental results show that the WGS reaction under FTS reaction conditions is far from equilibrium. Two types of WGS mechanisms are investigated. One is the formate mechanism, and the other is the direct oxidation mechanism. It is found that the formate mechanism is better in fitting experimental data than the direct oxidation mechanism over the Fe-Mn catalyst under the FTS reaction conditions. The optimized kinetics model with formate intermediate dissociation as the rate-determining step (RDS) can fit the WGS experimental results well. The simplified WGS kinetics model can easily be used for industrial modeling applications. (c) 2005 Elsevier Ltd. All rights reserved.