Three-armed poly(ethylene oxide) (PEO) star polymers were synthesized from tris(p-isocyanatophenyl)-methane and methoxypoly(ethylene glycol)s of molecular weight 350, 750, and 1900. The polymers exhibit lower critical solution temperature behavior in water, the cloud points decreasing linearly with the molal concentration of added salts. Surface tension and viscometric data reveal the formation of micelles and the possible existence of smaller aggregates prior to micellization. Similar amphiphilic PEO star polymers but with a larger hydrophobic core were found to strongly interact with the optical probes coomassie brilliant blue (CBB) and 1-pyrenebutyrate (PB-), both below and above the critical micelle concentration (cmc). Klotz-type binding plots reveal clear transition points which for PB- are close to the cmc of the star polymers. The binding data confirm that small aggregates may be present below the cmc of the stars.