The kinetics of the water gas shift reaction was investigated in supercritical water using the Raman spectroscopic method for in situ quantitative measurement of reactants and relevant products in a stopped now type reactor. The reaction proceeded according to the stoichiometric reaction as CO + H2O --> CO2 + H-2, whose rate was first order on the concentration of CO. The dependence on the reactant water was 1.5 +/- 0.1 in the water density range from 4 to 25 mol dm(-3) at a temperature of 380degreesC. The steady state concentration of formic acid was in the order of 10(-4) against the initial concentration of CO and dependent on a 1.2 +/- 0.3 order on the water density. These findings evidence that the rate determining step of the water gas shift reaction is the formation process of formic acid, which subsequently dissociates to CO2 + H-2 at a much larger reaction rate. The slightly larger dependence than unity on the water density in the reaction rate as well as in the steady state concentration of the intermediate formic acid suggests that the transition state structure of the formic acid formation process resembles a certain kind of complex formed between a CO molecule and several water molecules such as suggested by Melius et al. (1990) by ab initio calculations.