A novel iridescent gel phase of surfactant has been found in a ternary mixture of a double-chain surfactant (triethanolammonium dihexadecylphosphate), water, and ethanol. The structure of the iridescent gel phase has been studied mainly by X-ray diffraction and ultraviolet and visible light reflection techniques and determined to be lamellar, having the spacing distance of a submicrometer. The color appearance of the solutions results from diffraction of visible light by the lamellar structure of bilayer membranes. It is particularly interesting to note in this system that the iridescent color can be changed with temperature (0-20 degrees C), ethanol concentration (15-48 wt %), and surfactant concentration (1.0-2.0 wt %) as well. The iridescent color begins to shift to the blue side at a certain critical temperature when the temperature is elevated. Turbidity of the solution also starts to increase at the same critical temperature. When these critical temperatures are connected as a function of surfactant concentration, a curve similar to that of the lower critical solution temperature type phase diagram is obtained. In the two-phase region of this diagram, the iridescent color changes with temperature, owing to the concentration change of the condensed (lamellar) phase, and does not change at constant temperature even when the surfactant concentration is altered. These novel phenomena in the present iridescent surfactant systems can be ascribed to the phase separation into a lamellar structure and disordered bilayers of the surfactant gel phase. This order-disorder phase transition is substantiated by the freeze-fracture electron microscopic technique. This new type of phase separation phenomenon gives us a challenging fundamental problem on the interactions between colloidal particles.