The surface tension of aqueous solutions of 2-pyrrolidinone, 1-methyl-2-pyrrolidinone, and 1-ethyl-2-pyrrolidinone has been measured at temperatures from (20 to 50) degrees C over the whole range of concentrations. The surface tensions of these aqueous binary mixtures show more concentration dependence on the water-rich side. The addition of a small amount of N-alkyl-2-pyrrolidinone drastically reduces the surface tension of water. This effect is related to not only the presence of hydrophobic groups that tend to remain at the air-water interface but also the influence of the complex interactions between N-alkyl-2-pyrrolidinones molecules on the surface tension.