The association behavior of a triblock ethylene oxide (E) and butylene oxide (B) copolymer with a very long middle E block (B(12)E(260)B(12)) in aqueous solution has been investigated by means of laser light scattering. The flower-like closed association behavior of BEB-type triblock copolymers in aqueous solution can be altered to form open, branched, associated structures because of the very long E blocks. The B(12)E(260)B(12) triblock forms flower-like micelles in a closed association process with a critical micelle concentration of 0.056, 0.040; and 0.033 mg/mL even at fairly low temperatures of 10.0, 20.0, and 25.0 degrees C, respectively. At room temperature, B(12)E(260)B(12) can form large secondary associated structures which coexist with the flower-like micelles at concentrations of only 1-2% (w/v). The secondary associates increase in both amount and size with increasing concentration and increasing temperature, producing a highly viscous fluid even for a 4% aqueous solution of B(12)E(260)B(12).