The solubility of hydrogen in an ionic liquid (IL) is important for hydrogenolysis and hydrogenation used in biomass transformations. In this work, we formulate an enhancement ratio (ER) for H-2-CO2-IL systems that is defined in terms of the ternary system H-2 molality, the binary H-2 molality at constant temperature and the isofugacity condition. Literature data were used to demonstrate the utility of the definition for ER and to develop a predictive model for ionic liquids that dissolve cellulose. The ER is greater than one when CO2 can be used to increase Hy solubility in the IL. Although the solubility of H-2 in many IL can be enhanced by CO2, for some ILs the presence of CO2 reduces Hy solubility in the IL. The ER for [hmirn][Tf2N] was found to be less than one at low CO2 concentrations. Temperature affected the ER of ionic liquids in different ways with some IL exhibiting maximum hydrogen solubility ([bmim][PF6]) and others having an ER that monotonically increased with temperature ([Tf(2)N1-containing IL). Correlation of the ER with literature data is proposed in terms of Henry's constant of Hy and CO2 and the solubility parameter of the ionic liquid. The correlation was applied to six ionic liquids at conditions of 313-453 K and at CO2 molalities up to 5 mol/kg to give a sum of squares of the residuals of 1.4 x 10(-3) in Delta m(H2). Solubility enhancement ratios for Hy gas by CO2 for ionic liquids that dissolve cellulose were estimated with the predictive model to be as high as 1.2 per unit molality CO2 in the IL. Vapor-liquid equilibrium measurements of mixed gases (H-2-CO2, CO-CO2, O-2-CO2) with ionic liquids that dissolve biomass are urgently needed to promote development of methods for transforming biomass into biofuels and chemicals. (C) 2014 Elsevier B.V. All rights reserved.