The phase behavior and structure of aqueous solutions of a triblock copolymer of ethylene oxide and 1,2-butylene oxide, E16B10E16, have been studied as a function of polymer concentration (5-75 wt %) and temperature (5-95 degrees C) using tube inversion, rheometer, polarized light microscopy, and small-angle X-ray scattering. Sol-gel and gel-sol boundaries were determined by rheometry and tube inversion methods. Polarized Light microscopy and small-angle X-ray scattering were used to identify the lyotropic phases. A hexagonal mesophase was found for the stiff gel region (40-52 wt %). A lamellar phase was located in the gels of concentrated solutions (greater than or equal to 54 wt %) by using polarized light microscopy and small-angle X-ray scattering. Furthermore, solutions with lamellar structure were detected by small-angle X-ray scattering for the mobile phases of the concentrated solutions (>44 wt%). Small-angle X-ray experiments also showed that the concentrated cloudy sols (>40 wt%) at high temperature had aggregates with lamellar structure. Two soft gel regions were defined : the soft gel region I concerning the interaction of spherical micelles, which was only detected with a rheometer, was found at moderate concentrations (20-40%), and the soft gel region II concerning the arrangement of rodlike micelles, which was defined with a rheometer and also the tube inversion method, was found in the concentration region 40-49 wt % before the formation of the hard gel. The transition of spherical micelles to rodlike micelles was also investigated by rheometer at high temperatures for low concentrated solutions. The detailed phase diagram from 5 to 75 wt % solutions of the copolymer is presented.