Gates and co-workers had previously noted that supported metal clusters (e.g. Ir4 and Rh6) prepared from organometallic precursors typically possess much longer bond lengths than expected. Vayssilov and Rosch determined that these anomalous bond lengths are due to the presence of adsorbed hydrogen likely present from “reverse” spillover of hydrogen from support hydroxyl groups onto to the cluster. Here we expand on the work of Vayssilov and Rosch to examine the effect of the support identity on reaction enthalpy of this process to determine if this phenomenon is, in fact, general as the previous study was limited to a single zeolite model. In our examination of various supports (Al2O3, SiO2, MgO, TiO2), we find considerable variation between substrates. Results are compared in the context of the ease of hydroxylation of these supports.