Two series of hydrophobe-modified, ethoxylated urethane (HEUR) polymers were synthesized. The first series was prepared by the step-growth (S-G) polymerization of poly(oxyethylene) (POE) of M(n) = 6000, with a slight excess of different aliphatic diisocyanates in a 4.2 to 3.2 mole ratio to produce a S-G polymer with M(n) of similar to 20 000. The terminal isocyanate groups were reacted with alkylamines of different hydrocarbon chain lengths. The second series was produced by reacting POE of M(n) = 20 000 with a large excess of the diisocyanate to produce POE with terminal isocyanate functionality, followed by reaction with alkylamines. The molecular weight distribution of the second series was narrow compared to the first series, prepared by S-G polymerization. The hydrocarbon chain length of the alkylamine, the coupling diisocyanate, and the molecular weight distributions were systematically varied by the proper choice of synthetic conditions. The changes in structures were correlated with the rheological behavior of aqueous HEUR solutions. Their aqueous solution behaviors also were evaluated in the presence of anionic and nonionic surfactants. HEUR thickeners prepared with bis(4-isocyanatocyclohexyl)methane (H(12)MDI) were more effective in building viscosity than HEURs prepared from hexamethylene diisocyanate (HDI). In general, HEURs, with a narrow molecular weight distribution, gave higher aqueous solution viscosities than their corresponding broad molecular weight distribution counterparts at equal concentrations. Approximately twice the concentration of the broad molecular weight distribution HEUR with terminal C12H25-H(12)MDI hydrophobes was required to achieve comparable viscosity with the narrow molecular weight distribution HEUR. Despite the differences in concentration, the storage and loss moduli responses were similar. Solutions with small terminal alkyl groups were predominantly viscous in their viscoelastic response. The size of the "effective" terminal alkyl groups dominated aqueous solution rheological responses of both narrow and broad molecular weight distribution HEURs.