We have developed a ternary equation of state (EOS) model for the N2O/CO2/1-butyl-3-methylimidazolium tetrafluoroborate abmim] [BF4]) system in order to understand separation of these gases using room-temperature ionic liquids (RTILs). The present model is based on a generic RK (Redlich-Kwong) EOS, with empirical interaction parameters for each binary system. The interaction parameters have been determined using our measured VLE (vapor-liquid equilibrium) data for N2O/[bmim][BF4] and CO2/[bmim][BF4 and literature data for N2O/ CO2. The binary EOS models for the N2O/[bmim][BF4] and CO2/[bmim] [BF4]systems correctly predicted the liquid-liquid phase separation found in VLLE experiments. The validity of the ternary EOS model has been checked by conducting VLE experiments for the N2O/CO2/[bmim][BF4 system over a range in temperature from 296 to 315 K. With this EOS model, solubility (VLE) behavior has been calculated for various (T, P, and feed compositions) conditions. For both large and small N2O/CO2 feed ratios, the N2O/CO2 gas selectivity [alpha(N2O/CO2) = (y(N2O)/x(N2O))/(y(CO2)/x(CO2))] is alpha = 1.4-1.5, compared with (alpha = 0.96-0.98) in the absence of ionic liquid. While the concentration of the ionic liquid does not affect the selectivity, the addition of an ionic liquid provides the only practical means of separating CO2 and N2O.