A mathematical model for a non-isothermal fixed bed reactor with supported ruthenium catalyst for the conversion of synthesis gas was developed and tested with experimental results. A two-dimensional pseudo-homogeneous model with the well-known power law for the reaction rate was found to be suitable after consideration of all the possible transport mechanisms occurring under conditions favouring the formation of useful gaseous, liquid and solid (waxes) hydrocarbons. The reaction rate was evaluated by a non-linear regression technique in order to demonstrate the validity of the equation and the associated constants for a fixed bed. It was found that the hydrogenation and/or polymerisation reactions occurring along the length of the fixed bed reactor influenced the reaction rate constants for the conversion of carbon monoxide conversion, whereas, the reaction rate constants for the formation of methane remained unaffected when compared with results obtained from a differential bed reactor. Reaction rate constants are also reported for the formation of a light gas fraction (C-2 to C-4).