Oxyethylene/oxybutylene block copolymers E41B-8, E21B-8E21, and B4E40B4 were prepared by sequential anionic polymerization and characterized by gel permeation chromatography and NMR spectroscopy. Their association behavior in aqueous solution was investigated by surface tension and static and dynamic light scattering. The diblock copolymer E41B-8 and the triblock copolymer E21B-8E21 associated by closed processes to form micelles, those of the diblock copolymer being much larger. The triblock copolymer B4E40B4 associated by a process having characteristics of open association. A low-temperature clouding-clearing effect was noted for solutions of copolymer E21B-8E21. Thermally-reversible sol --> gel --> sol transitions were observed on heating or cooling concentrated solutions of diblock copolymer E41B-8, but concentrated solutions of the two triblock copolymers did not gel.