The phase behavior of thirty mixtures of Appalachian crude oil and carbon dioxide are described. Stock tank oils from the Hilly Upland field in north central West Virginia, and the Bath field in north central Ohio are examined in the presence of equimolar amounts of CO/sub 2/. The effects of small amounts of contaminant gases are probed using methane, nitrogen, hydrogen sulfide and sulfur dioxide. Additional changes due to the presence of brine are studied. Data was collected in single-contact phase equilibrium experiments using a high pressure windowed condensate PVT cell. The volumetric behavior of sample mixtures was monitored. Total sample and individual phase volumes were recorded up to pressures of 3600 psi, and at temperatures above and below the CO/sub 2/ critical temperature. Saturation pressures and the extent of oil swelling were determined from the PVT measurements. Significant results show that the phase equilibria determined for CO/sub 2/-Appalachian crude oil mixtures generally emulate that reported for Western crude oil systems. Although swelling indices were generally insensitive to small additions of CH/sub 4/, N/sub 2/ or H/sub 2/S; swelling is promoted by SO/sub 2/ in the absence of brine. Bubblepoint pressures were notably affected by the additives studied. Changes in bubblepoint pressure induced by small additions of additive gas correlate inversely with the gas critical temperature. While some precipitate formation is reported for CO/sub 2/-Western crude oil mixture, no solid phase was observed for equimolar CO/sub 2/-Appalachian crude oil mixtures. Brine can stimulate precipitate formation, especially in the combined presence of H/sub 2/S or SO/sub 2/. Finally, phase behavior and displacement results both suggest that CO/sub 2/ flood oil recovery efficiencies will be improved by small additions of H/sub 2/S or SO/sub 2/, but reduced by small additions of CH/sub 4/ or N/sub 2/.