The temperature dependent properties of aqueous solutions of a triblock copolymer of ethylene oxide (EO) and propylene oxide (PO), with a measured composition EO(28)PO(48)EO(28), have been investigated by use of theological techniques and scanning calorimetry at temperatures between 5 and 85 degrees C and concentrations up to 40 wt %. At low temperatures a unimer-micelle equilibrium is established, which is shown to be due to the interactions between the PO block and water. Complex viscoelastic properties are observed with increasing temperatures. Two sol-to-gel transitions are observed with increasing temperature followed by phase separation. The low-temperature gel, which has a large elastic storage modulus, consists of close-packed spherical micelles. This gel state occurs above a lower critical point of 32 degrees C and 24.5 wt %. Its large elastic shear modulus is attributed to increasing exposure to water of hydrophobic PO groups in the micellar core under strain. The high-temperature gel, which is formed at concentrations as low as 1-2 wt %, has a smaller elastic modulus which is attributed to hindered rotation of overlapping rodlike micelles. A third gel state, which is a two-phase region, is formed at concentrations above 28%. The complex formation of micelles and gels is summarized in a concentration-temperature diagram.