The aggregation and phase behavior in water of several triblock copolymers of poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene)[(EO)x(PO)y(EO)x] has been studied. The y-values of the compounds ranged from 16 to 70 and the x:y ratios from 0.1 to 2.5. All studied compounds form micelles and lyotropic liquid crystalline phases. For a constant temperature the critical micelle concentrations (cmc) of the compounds decrease exponentially with y. The energy increment for the transfer of a PO group from the aqueous to the micellar state is about (0.25 +/- 0.05) kT. The cmc values for all compounds decrease strongly with increasing temperature. As a consequence the solutions undergo a monomer-micelle transition for constant concentration and increasing temperature. This micellization process is associated with a large endothermic heat which is linearly dependent on the size of the PO block. It is concluded that this heat is due to the dehydration of the PO groups, and it is called the heat of micellization DELTAH(m). For most of the studied compounds DELTAH(m) = 3.0 +/- 0.5 kJ/mol for one PO group. The large change of the cmc values with temperature can quantitatively be explained by the large DELTAH(m) values. The sequence of the lyotropic mesophases is mainly determined by the x:y ratio. Systems with x:y > almost-equal-to 0.5 form spherical micelles for c > cmc. The size of the micelles is independent of the concentration and temperature, if the temperature is about 20-degrees-C above the micellization temperature T(m); in a transition region around Tm the micellar size increases strongly with temperature. Below T(m) or the cmc only monomeric block copolymer molecules are present in the solution. At higher concentrations and temperatures solutions with spherical micelles form in a first-order transition a transparent, optically isotropic, highly viscous, and elastic cubic phase. The formation of this cubic phase can be understood by hard-sphere interaction between the aggregates. With further increasing concentrations transitions to hexagonal and to lamellar phases are observed. Samples with a smaller hydrophilic EO block, i.e., with x:y almost-equal-to 0.25, usually form a hexagonal phase as the first liquid crystalline mesophase, while for systems with ratios xy almost-equal-to 0.15 a lamellar phase is found as the first mesophase; samples with x:y much less than 0.1 are no longer soluble in water. The lyotropic mesophases show also a thermotropic behavior; i.e., reversible transitions cubic --> hexagonal --> lamellar or from isotropic solutions to mesophases occur at constant block copolymer concentration with increasing temperatures. The mesophases usually melt at temperatures below 100-degrees-C to systems consisting of one or more isotropic liquid phase.