Time-resolved small-angle X-ray diffraction experiments have been carried out to investigate the transformation from the cubic gyroid Ia3d to the cubic diamond Pn3m phase. The transition was induced by rapid addition of water to a monoolein/water system changing the initial water concentration of 26% w/w to excess of water conditions. The transformation proceeds in two steps: at the beginning, the two cubic mesophases have a strongly differing lipid/water composition and are accompanied by an intermediate phase. This phase vanishes as the ratio of cell parameters exceeds 1.50. Thereafter, the transition continues by a direct nearly isoareal transformation of the cubic minimal surfaces. Lateral area per lipid molecule, molecular shape and principle curvatures are about the same in this regime.