A kinetic model for Fischer-Tropsch synthesis (FTS) in both conventional reaction media (gas-phase) and non-conventional media (near-critical and supercritical solvent media) was derived from experimental data generated from a commercial alumina supported cobalt catalyst (15% Co/Al2O 3). The model was developed based on partial pressures assuming ideal gas phase as well as based on fugacity coefficients to account for the non-ideality in the near critical and supercritical phase. The estimated kinetics parameters from the fugacity-based model were compared to the kinetics parameters obtained from the ideal gas assumption using the partial pressure based kinetic model. Similarly, the fugacity-based model successfully predict the methane formation rates for gas phase as well as near critical and supercritical phase FTS. It was observed that the fugacity-based models more accurately predicted the carbon monoxide consumption rate in gas-phase as well as near critical and supercritical FTS conditions. Our qualitative conclusion is that fugacity-based model will also do a better job than the partial pressure based model in predicting the chain growth probability of the cobalt catalyst since it can better account for the non-ideality of the reaction in the gas-phase FTS (high pressures operation) and in the near-critical and supercritical phase.