An in situ X-ray observation of the postspinel transformation kinetics was made using intense synchrotron radiation. We confirmed that Mg2SiO4 spinel transforms into fine lamellae of SiO2 stishovite and periclase, and/or MgSiO3 ilmenite and periclase as an intermediate step in the postspinel transformation. These metastable assemblages eventually disappear and form the stable assemblages of MgSiO3 perovskite and periclase. Initial grain size just after the postspinel transformation drastically changes with overpressure. Viscosity of the subducting slab into the lower mantle, which is thought to be deformed by grain-size-sensitive creep, would depend on overpressure needed for the postspinel transformation at geological time scale. ¿ 2000 American Geophysical Union