The solubility of water and carbon dioxide in the ionic liquid 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]) is computed using atomistic Monte Carlo simulations. A newly developed biasing algorithm is used to enable complete isotherms to be computed. In addition, a recently developed pairwise damped electrostatic potential calculation procedure is used to speed the calculations. The computed isotherms, Henry's Law constants, and partial molar enthalpies of absorption are all in quantitative agreement with available experimental data. The simulations predict that the excess molar volume of CO2/ionic liquid mixtures is large and negative. Analysis of ionic liquid conformations shows that the CO2 does not perturb the underlying liquid structure until very high CO2 concentrations are reached. At the highest CO2 concentrations, the alkyl chain on the cation stretches out slightly, and the distance between cation and anion centers of mass increases by about 1 angstroms. Water/ionic liquid mixtures have excess molar volumes that are also negative but much smaller in magnitude than those for the case of CO2.