The synthesis, characterization, and solution theology of well-characterized hydrophobically-modified, ethoxylated urethane (HEUR) water-soluble polymers are described for two types of model HEURs : linear poly(oxyethylene) of M(n) = 26 200 with terminal hydrophobes of different sizes, and terminal hydrophobe groups separated by smaller oxyethylene spacing around a larger internal hydrophobe. In the first series, a terminal isocyanate telechelic prepolymer of poly(oxyethylene) is prepared and reacted with amine and alcohol containing hydrophobes of variable size. In the second, terminal hydrophobe HEURs varying in geometry around an internal hydrophobe are synthesized by a one-step addition of an ethoxylated nonylphenol surfactant to a diisocyanate or an isocyanato functional biuret or isocyanurate. Low shear rate viscosities and oscillatory responses of HEUR solutions are examined, alone and in the presence of anionic and nonionic surfactants. HEUR solutions exhibit a maximum in viscosity with increasing surfactant concentrations with both anionic and nonionic surfactants. The viscoelasticity of the solution in the area of the viscosity maximum and factors influencing the magnitude of the viscosity increase are addressed. The surfactant concentration necessary to achieve the viscosity maximum is observed to depend on the concentration and architecture of the HEUR as well as the structure of the surfactant. Differences in the phase separation behavior of aqueous solutions also are observed.