Cloud point curves and conjugate coexistence curves for the quasi-binary water systems of ethyl(hydroxyethyl)-cellulose (EHEC) and its hydrophobically modified analogue (HMEHEC) have been determined. The cloud point curves are remarkably independent of the polymer concentration up to as high as 20 wt % polymer, and they are clearly different from the coexistence curves. This is interpreted as an effect of the multicomponent nature of the polymer. Hydrophobic modification of EHEC lowers the cloud point by approximately 15 degrees C. On heating above the cloud point, EHEC redistributes to the concentrated phase, but, however, a substantial amount of the polymer remains in the dilute phase even far above the cloud point. For HMEHEC, essentially all polymer is found throughout in the concentrated phase. Addition of the anionic surfactant, sodium octylbenzenesulfonate (SOBS), affects the phase behavior. Small amounts of SOBS cause a swelling of the concentrated phase for both EHEC and HMEHEC, due to an electrostatic repulsion between polymer-surfactant aggregates. On further addition of SOBS, a dissolution of polymer from the concentrated phase is observed for EHEC. For HMEHEC, the surfactant binding swells the concentrated phase until the one-phase region is reached.