Three copolymers of ethylene oxide (EO) and 1,2-butylene oxide (BO) were prepared by anionic polymerization to have similar molar masses (M(n) approximate to 7000 g mol(-1)), molar mass distributions (M(w)/M(n) < 1.1) and compositions (80 mol% oxyethylene) but different chain structures. A triblock copolymer was prepared by sequential copolymerization of EO followed by BO, and two statistical copolymers were prepared from mixed monomers, either to low (15%) or to high (93%) conversion. The disparity in reactivity ratios in the EO/BO copolymerization resulted in pronounced composition drift along the chains of the high conversion statistical copolymer, giving it the character of a block copolymer. This composition profile was checked by C-13 n.m.r. spectroscopy. The association behaviour of the statistical copolymers in aqueous solution at 25 degrees C was studied by dynamic and static light scattering. The low conversion statistical copolymer remained in its molecular state across the concentration range studied (up to 100 g dm(-3)), but the high conversion statistical copolymer associated into molecular clusters and micelles. Cloud point curves were determined for solutions of the three copolymers. The effect of association on phase behaviour is discussed.