The micellization thermodynamics and micelle structural parameters for polystyrene-b-poly(ethylene/butylene)-b-polystyrene copolymers (SEBS) in n-octane were studied. This solvent is selective for the middle poly(ethylene/butylene) block of the copolymer. The copolymer samples have similar chemical composition and different molar mass. Standard thermodynamic functions of micellization (Delta G degrees, Delta H degrees and Delta S degrees) were determined by light scattering. All the functions showed more negative values for the SEBS copolymer with a larger molar mass. The structural parameters of micelles formed by different triblock copolymers were determined from static light scattering and viscosity measurements. Micelle molar mass, association number and viscometric hydrodynamic radius increase with the length of the copolymer chain for a constant copolymer composition; however, the radius of gyration decreases. Finally, the results found for SEBS micelles in n-octane are compared to those found for SEBS micelles in 4-methyl-2-pentanone. The different behaviour found supports the idea that the micelles of a triblock copolymer in a selective solvent of the middle block will show a shell formed by loops of the middle block in order that the two outer blocks go into the micelle core and even some poorly solvated outer blocks could be extended out of the micelle.