An improved reservoir fluid characterization procedure for miscible gas floods that can more accurately represent the interaction of flow and phase behavior is presented. The approach for two displacements, an eleven-component CO2 flood and a twelve-component enriched gas flood, are demonstrated. The method-of-characteristic (MOC) theory is used to determine the minimum miscibility enrichment (MME) (or pressure (MMP)) of both lumped and unlumped models. By tuning to the MME/MMP, fewer pseudocomponents are required to characterize the fluid than with conventional tuning methods. For the cases studied, fluids lumped to as few as four pseudocomponents can match well the composition profiles and oil recoveries of the unlumped models. Four pseudocomponents are sufficient to model the 12-component enriched-gas drive, saving a significant amount of computation time in compositional simulation. For the 12-component enriched-gas flood, the swelling data did not improve the fluid characterization of the lumped four-component model. For the CO2 gas flood, there is no significant difference between models tuned to match all PVT data including the swelling data and that tuned to match the MMP and PVT data, without matching the swelling experiment. This is an abstract of a paper presented at the SPE Europe/EAGE Annual Conference (Madrid, Spain 6/13-16/2005).