Phase behavior of ethylene oxide (EO)-propylene oxide (PO)-ethylene oxide triblock copolymers in water has been investigated, by using 2 H NMR and polarizing microscopy, as functions of both concentration and temperature. Seven copolymers, having wide variations of composition and molecular weight, are used. It has been demonstrated that formation of liquid crystalline phases above certain polymer concentrations is a rather common feature for the EO-PO-EO triblock copolymer-water systems. The liquid crystalline phases formed are cubic, hexagonal, and lamellar phases. A cubic phase which may possess a bicontinuous-type structure has been identified. Four factors are important for controlling the phase behavior. They are molecular weight, polymer chemical composition, temperature, and concentration. A minimum molecular weight of the copolymer is needed in order to form the liquid crystalline phases. Moreover, a close relationship between the cloud point of the copolymer and the mesophase structures has been indicated