Two oxyethylene/oxybutylene block copolymers of similar chain length and composition, E(38)B(12) and E(21)B(11)E(21), were prepared by sequential anionic polymerization and characterized by gel permeation chromatography and NMR spectroscopy. Their association in dilute aqueous solution was investigated by static and dynamic light scattering. Both copolymers associated by closed processes to form micelles, those of the diblock copolymer being the larger. The increment per B unit in the standard Gibbs energy of micellization of diblock E(m)B(n) copolymers was found to be 2.0 kJ mol(-1). The critical micelle concentrations of oxyethylene/oxybutylene (E(m)B(n) and E(m)B(n)E(m)), oxyethylene/alkyl (E(m)C(n)), and oxyethylene/oxypropylene (E(m)P(n)E(m)) copolymers are compared, and the effect of chain architecture (diblock and triblock) on the association of diblock and triblock copolymers is discussed.