The effect on the phase behavior of an aqueous solution of a nonionic polymer, with a reduced solubility at elevated temperature, of the addition of a surfactant or a second polymer (ionic or nonionic) is studied both theoretically (Flory-Huggins theory) and experimentally. Particular interest is focused on the effect on the clouding temperature of the polymer-water system caused by the addition of the surfactant. Good agreement is found between theory and experiment, modeling the surfactant as a second polymer, and the phase behavior is found to depend strongly on both the effective polymer-solvent and the polymer-surfactant interactions. In particular, it is shown that even a fairly weak effective attraction between the clouding polymer and the added macromolecule or micelle may lead to a lowering of the clouding temperature. It is also shown for the case where the added polymer or surfactant is ionic that addition of salt will drastically change the phase behavior; this is discussed in terms of the additional entropy associated with the counterion distribution.