Time-resolved X-ray diffraction was used to monitor structure changes during a partial lamellar liquid crystal (smectic A)/lamellar gel (smectic B) transition in lipid multilayer vesicles in response to small pressure oscillations covering 4 decades in frequency. The data suggest that hydrocarbon chain ordering/disordering is tightly coupled to changes in lamellar unit cell length that occur during the phase change and are consistent with a bidirectional layer-by-layer transition mechanism best described by the Kolmogorov-Avrami kinetic model having an effective dimensionality of 1.