Binary mixtures of the poly(oxyethylene) surfactant C(12)EO(2) and water have been investigated using optical microscopy and time-resolved X-ray diffraction during temperature scans, At concentrations in the range from 48 to 70 wt % of surfactant a thermal sequence from lamellar L(alpha) to cubic Ia3d to cubic Pn3m to L(2) was found upon heating. The geometrical parameters of the phases such as thickness of the hydrocarbon core of the lamellar phase and length and diameter of the rods forming the cubic structures were calculated for such conditions. In the lamellar L(alpha) phase the surface area per molecule and the thickness of the hydrocarbon core showed Little sensitivity to concentration. The thickness of the ethoxy groups was estimated to 1 nm. At high concentrations and temperatures near the cloud point, equilibrium conditions are difficult to reach, leading to the formation of cubic phases with apparently the same structure but different thermal behavior. One is insensitive to temperature changes while the other shrinks upon heating. The transition between the Ia3d and Pn3m cubic phases has the characteristics of a Bonnet transformation.