The phase behavior at different temperatures was studied for aqueous mixtures of the polymers dextran T70 or poly(ethylene oxide) 40000 (PEO) with the surfactants penta(ethylene oxide) dodecyl ether (C(12)E(5)), octa(ethylene oxide) dodecyl ether (C(12)E(8)), or n-octyl-beta-D-glucopyranoside (C(8)G(1)). At sufficiently high concentrations, a segregative two-phase separation, with the polymer and the sufactant being enriched in different phases, was found for all polymer/surfactant couples. The segregation became stronger with increasing size of the surfactant micelle. Micellar growth was considered to be the major factor governing the rapid growth of two-phase area with increasing temperature in mixtures involving C(12)E(5) (at all temperatures) and C(12)E(8) (above room temperature). In contrast, under conditions of approximately constant micellar sizes, the two-phase area decreased slightly with increasing temperature. Short-range interactions between the surfactant headgroups and the polymers were found to be important for the extent of the segregation. The chemically similar couples dextran/C(8)G(1) and PEO/C(12)E(8) were more miscible than the dissimilar couples dextran/C(12)E(8) or PEO/C(8)G(1), but the opposite trend was found for the mixtures involving C(12)E(5).