The structural transformations of a MgO-attached [Ru6C] cluster catalyst during carbonylation under CO/H-2 and decarbonylation under vacuum were studied by energy-dispersive X-ray absorption fine structure (DXAFS). The DXAFS study revealed that the carbonylation of [Ru6C]/MgO to [Ru6C(CO)(11)]/MgO proceeded via two intermediates with the same composition [Ru6C(CO)(6)] with different Ru-Ru bond distances (0.265 and 0.269 nm). First, six CO molecules coordinate to a [Ru6C] framework, and second, the Ru-Ru length expands from 0.265 to 0.269 nm. Finally, five CO molecules coordinate to the [Ru6C(CO)(6)] cluster to produce [Ru6C(CO)(11)] species, where the Ru-Ru distance is further elongated to 0.271 nm. In the decarbonylation process, the [Ru6C(CO)(6)]/MgO is partially decarbonylated to form [Ru6C(CO)(11)]/MgO accompanied with a decrease of Ru-Ru length from 0.271 to 0.269 nm. Then the complete decarbonylation occurs to form [Ru6C] species, keeping the Ru-Ru distance at 0.269 nm. Finally, the cluster framework shrinks to recover the original [Ru6C]/MgO with the Ru-Ru distance of 0.265 nm. There were definite time lags between the first carbonylation and the cluster framework expansion (2-4 s, depending on the temperature) and between the last decarbonylation and the cluster framework shrinkage (3-5 min, depending on the temperature). The activation energies for the structural changes in the Ru cluster framework in every transformation step under CO/H-2 (423-523 K) and vacuum (573-623 K) were estimated by the time-resolved DXAFS analysis. This sort of study is the first to provide structural kinetics of catalytically active metal sites at oxide surfaces.