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Time-resolved X-ray diffraction was employed to monitor transitions, induced by pressure jumps, between inverse lyotropic liquid crystalline phases of the mixed lipid system lauric acid/dilauroylphosphatidylcholine (2:1 molar ratio) in water. We studied transitions between the gyroid, , and the double diamond, , inverse bicontinuous cubic phases and between and the inverse hexagonal, H_II, phase at a fixed water composition of 50 wt %. The transition → occurred within the 0.5 s time resolution of our X-ray measurements. This transition occurs without any change in water content within the mesophase but with a small change in the total topology. The transition → ( + H_II + excess water) was observed at 59.3, 62.8, and 66.9 °C. The process appeared to follow first-order kinetics, with rate constants of 0.11, 0.17, and 0.26 s^-1, respectively. The reverse transition occurred much more quickly, being completed in …