Based on the common belief that water inhibits the intrinsic Fischer-Tropsch ( FT) reaction rate in the iron-FT synthesis, water is included in almost all iron- FT kinetic expressions. A new rate expression is now proposed where vacant sites, CO, and water are all included in the denominator. This model was evaluated with data from various historical experimental studies. In all cases it was found that the effect of water is not statistically significant and should therefore be omitted from the model. The new model describes the historical data more accurately than some other popular rate equations. To validate these conclusions, new experimental data were measured in a well- mixed slurry reactor in the absence of mass transfer limitations. The experimental methodology employed ensured that the iron- FT catalyst did not suffer measurable deactivation. It was confirmed that there is no basis for including water in the denominator of the new rate equation and that it is more accurate than the rival models considered.