The associative behavior of a triblock copolymer polystyrene-b-poly(ethylene/butylene)-b-polystyrene dissolved in solvent mixtures of n-octane sind 4-methyl-2-pentanone has been studied. When the solvent composition was varied, the solvent mixture shifted from a selective solvent of the end copolymer blocks (polystyrene) to a selective solvent of the middle block (poly(ethylene/butylene)). For solvent mixtures with a high 4-methyl-2-pentanone content (selective solvent of polystyrene), only one kind of particles has been detected in solution. These particles have a high molar mass and compactness and a small size (R-h approximate to 32 nm). However, for solvent, mixtures with a high n-octane content (selective solvent of poly(ethylene/butylene)), two types of particles of different size were detected. The smaller ones (R-h approximate to 22 nm) were considered as free copolymer chains and they were the more numerous in the solution. The larger ones (R-h approximate to 210 nm) were assigned to aggregates with a loose structure different to ordinary micelles. The n-octane/4-methyl-2-pentanone mixture with a 50% of ketone behaved as a nonselective solvent, and no aggregates were detected at 25 degrees C and at concentrations so high as 10 x 10(-3) g.mL(-1).