The evolution of the chemical state and change in the morphology of subnanometer cobalt clusters during the dehydrogenation of cyclohexene was investigated in terms of metal-support interactions. The model catalyst systems were prepared by deposition of size selected subnanometer Co27±4 clusters on various amorphous metal oxide supports (Al2O3, ZnO, and MgO), as well as on a carbon-based support (UNCD=ultrananocrystaline diamond). The reactivity, oxidation state, and sintering resistance of the clusters were monitored by temperature programmed reaction (TPRx), in situ grazing incidence X-ray absorption spectroscopy (GIXAS), and grazing incidence small angle X-ray scattering (GISAXS), respectively. The reactivity and selectivity of cobalt clusters show strong dependency on the support used, with clusters supported on UNCD possessing the highest activity at 300 °C. The evolution of the oxidation state of metal cluster during the reaction reveals that metal-support interaction plays a key role in performance of the subnanometer catalyst. A reversible assembly of clusters into a nanostructure which evolves with reaction temperature was observed on the MgO support.