It is commonly observed that macromolecular mixtures in a common solvent phase separate with each component enriched in its respective phase. In this case the segregative phase behavior between the anionic polymer polystyrene sulfonate and the anionic surfactant sodium dodecyl sulfate has been studied in water. In particular, the effect of additives on the phase behavior has been studied. The additives are sodium chloride, hexanol, octanol, and octane. With the exception of sodium chloride these additives are practically insoluble in mater but they still have a strong impact on the phase behavior. Salt, and alcohols such as hexanol and octanol, decrease the polymer-surfactant compatibility (i.e., the extent of the two-phase area increases). Addition of octane enhances the compatibility so that the two-phase area shrinks. The mechanism by which the additives act is to influence the micellar size. From viscosity measurements it has been found that addition of alcohols and salt induces micellar growth whereas addition of an alkane can reduce the micellar size.