The high-pressure phase behavior of the carbon dioxide + 1-methyl-2-pyrrolidinone and carbon dioxide + 1-ethyl-2-pyrrolidinone systems has been studied. Pressure- composition isotherms were measured for binary mixtures of the carbon dioxide + 1-methyl-2-pyrrolidinone system at (45, 65, 85, 105, and 125) degreesC and pressures up to 250 bar and for the carbon dioxide + 1-ethyl-2-pyrrolidinone system at (40, 60, 80, 100, and 120) degreesC and pressures up to 232 bar. The solubility of 1-methyl-2-pyrrolidinone and 1-ethyl-2-pyrrolidinone for the carbon dioxide + 1-methyl-2-pyrrolidinone and carbon dioxide + 1-ethyl-2-pyrrolidinone systems increases as the temperatures increase at constant pressure. The carbon dioxide + 1-methyl-2-pyrrolidinone and carbon dioxide + 1-ethyl-2-pyrrolidinone systems exhibit type I phase behavior, characterized by a continuous critical line from pure carbon dioxide to the second component with a maximum in pressure. The experimental results for the carbon dioxide + 1-methyl-2-pyrrolidinone and carbon dioxide + 1-ethyl-2-pyrrolidinone systems have been modeled using the Peng-Robinson equation of state.