Results of a wide-ranging investigation of the scaling of the physical mechanisms of miscible floods are reported. Advanced techniques for analysis of crude oils are considered in Chapter 2. Application of supercritical fluid chromatography is demonstrated for characterization of crude oils for equation-of-state calculations of phase equilibrium. Results of measurements of crude oil and phase compositions by gas chromatography and mass spectrometry are also reported. They demonstrate that the presence of significant quantities of multi-ring aromatic compounds in a crude oil can increase substantially the minimum miscibility pressure for C0[sub 2] injection. The theory of development of miscibility is considered in detail in Chapter 3. The theory, is extended to four components, and sample solutions for a variety of gas injection systems are presented. The analytical theory shows that miscibility can develop even though standard tie-line extension criteria developed for ternary systems are not satisfied. In addition, the theory includes the first analytical solutions for condensing/vaporizing gas drives. In Chapter 4, the interactions of viscous fingering and permeability heterogeneity are examined. Results of flow visualizations are reported for unstable displacements in synthetic heterogeneous porous media. In addition, predictions of a particle-tracking simulator for the, same displacements are compared with the experimental observations. Both experiments and simulations show that even modest permeability heterogeneity can dominate the pattern of fingering in unstable displacements. Finally, a suite of unstable displacements in laboratory samples is interpreted with the particle-tracking simulator to examine the affects of changes in mobility ratio for displacements in which dispersion, heterogeneity, and viscous instability interact. In Chapter 5, the combined effects of capillary and gravity-driven crossflow are considered.