The phase behavior of N-cyclohexyl-2-pyrrolidone and a homologous series of N-alkyl-2-pyrrolidones mixed with water or with paraffins has been studied, and the effect of additives investigated. Depending upon the alkyl chain length, concentration, and temperature, the compounds can be either partially or completely miscible with water or paraffins. Several mixtures of the pyrrolidones and water show a lower consolute temperature (LCT). The LCTs can be changed to higher or lower temperatures by addition of salts, acids, and other compounds. Mixtures of the pyrrolidone compounds with paraffins, on the other hand, show an upper consolute temperature (UCT) behavior. The UCT decreases with the alkyl chain length of the pyrrolidones, but increases with the paraffin chain length. The miscibility of a given paraffin/pyrrolidone mixture can be improved by adding a longer chain pyrrolidone as third component.