Using gas-liquid chromatography, activity coefficients at infinite dilution (y(i,3)(infinity)) have been measured for 23 solutes (i) (alkanes, alkenes, cyclic hydrocarbons, aromatic hydrocarbons, ketones, ethers, aldehydes, esters, and halocarbons) in the solvents (3) 1-ethylpyrrolidin-2-one and 1,5-dimethylpyrrolidin-2-one. The measurements were carried out at three temperatures (298.15 K, 308.15 K, and 318.15 K). The obtained yi,3 values are compared with data For 1,3-dimethylimidazolidin-2-one, 1-octylpyrrolidin-2-one, pyrrolidin-2-one, and 1-methylpyrrolidin-2-one. The selectivities for benzene and the other organic solutes are presented at three temperatures (298.15 K, 308.15 K, 318.15 K), and the efficacy of the two entrainers for separating benzene from other organic substances is discussed. Furthermore, the experimental data are compared with the results of the modified UNIFAC equation (Dortmund).