The phase behavior and viscosity of aqueous solutions of two nonionic polymers, ethyl(hydroxyethyl)cellulose (EHEC) and the hydrophobically modified analogue (HM-EHEC), have been studied as a function of added cosolutes, ranging from alcohols with different chain lengths to salts with different anions and to substances usually referred to as denaturants. The change in phase behavior on addition of a cosolute is dominated by the general structure of the polymer chain and is negligibly influenced by the hydrophobic tails of HM-EHEC, as they are present in such a low number. Therefore, the cosolute induced changes in phase behavior are akin for the two polymers. However, the situation is different for added cosolutes which have a particularly strong affinity for the hydrophobic moieties of HM-EHEC and at a closer inspection differences in phase behavior between the two polymers can be seen on addition of long-chained alcohols. On the other hand, changes in viscosity on addition of cosolutes are generally much more pronounced for HM-EHEC as rheology probes network formation and association resulting from small changes in the interaction energy between the polymer chains. The observations, both phase behavior and viscosity, can be qualitatively understood in terms of the distribution of cosolute between bulk solution and polymer surface. As an example, addition of NaCl or a long-chained alcohol increases the viscosity more for HM-EHEC than for EHEC, while an addition of NaSCN or a short-chained alcohol rather decreases the viscosity, again more pronounced for HM-EHEC.