Biphasic ionic liquid/carbon dioxide (IL/CO₂) systems have been introduced as an efficient platform for homogeneously catalyzed reactions. The system has a number of unique properties such as no contamination of ionic liquid in the gas phase, increased gas solubility, tunable product separations, etc. Detailed phase equilibrium investigations are necessary to understand and engineer these reactions. This investigation reports the phase equilibria of reactants, products, and compressed gases in hydrogenation/hydroformylation reactions in biphasic systems with imidazolium ionic liquids and CO₂). Model reaction systems of the substrate 1-octene with hydrogen and with syngas are investigated. The sometimes-dramatic effects of phase equilibrium on the catalytic reaction engineering are discussed.