The transformation between two thermotropic bicontinuous cubic (Cub) phases has kinetically been determined by time-resolved X-ray diffraction. The system investigated was 1,2-bis(4'-n-alkoxybenzoyl)hydrazines (denoted as BABH-n, where n is the number of carbon atoms in the alkyl chain). Transitions from the Ia3d to Im3m phases of BABH-13 and from the Im3m to Ia3d phases of BABH-16 were induced by a temperature jump. Although the two Cub phases have quite different aggregation structures, the transitions between them appeared to occur without any diffractive intermediate states. A clear increase in the rate of transformation was observed upon increasing the extent of superheating, and the rate was slower than that reported previously for lyotropic systems. In addition, a comparison between the two transformations revealed that the rate of the Im3m to Ia3d transformation (of BABH-16) is ca. 5 times faster than that of Ia3d to Im3m (of BABH-13).