We present a detailed microkinetic model of the low temperature water gas shift (WGS) reaction catalyzed by Pt under realistic reaction conditions (250-300°C, 1 atm) and compare the model with experimental results. Thermodynamic and kinetic input parameters for the model are derived from periodic, self-consistent density functional theory (DFT) calculations based on ultrasoft Vanderbilt pseudopotentials and plane wave basis sets. The destabilizing effect of high CO surface coverage on the binding energies of surface species is quantified by DFT calculations and accounted for in the microkinetic model. Our model agrees well with measured experimental reaction rates and reaction orders. The calculated and experimental apparent activation energies are 29.5 kJ/mol and 31 kJ/mol, respectively. The model clearly shows that the most significant reaction channel proceeds via a carboxyl intermediate. Formate, which has been experimentally observed and thought to be the key WGS intermediate in the literature, is shown to act only as a spectator species.