Triblock copolymers of poly(ethylene oxide) (PEG) and poly(propylene oxide) (PPO) form lamellar mesophases at low temperatures. The order-to-disorder transition is closely related to the melting transition of the PEO subunit. When water is incorporated into the copolymer melt, a single phase is formed at high temperatures, which is possibly a lamellar mesophase, formed as a consequence of the hydrophobic PPO blocks. At low temperatures two phases are present, one of which is similar to the high temperature phase while the other is similar to the lamellar melt, but swollen up to 10%.