Hydrogenations of CO or CO2 are important catalytic reactions as they are interesting alternatives to produce fine chemical feedstock hence avoiding the use of fossil sources. Using monodisperse nanoparticle (NP) catalysts, we have studied the CO/H-2 (i.e., Fischer-Tropsch synthesis) and CO2/H-2 reactions. Exploiting synchrotron based in situ characterization techniques such as XANES and XPS, we were able to demonstrate that 10 nm Co NPs cannot be reduced at 250 degrees C while supported on TiO2 or SiO2 and that the complete reduction of cobalt can only be achieved at 450 degrees C. Interestingly, cobalt oxide performs better than fully reduced cobalt when supported on TiO2. In fact, the catalytic results indicate an enhancement of 10-fold for the CO2/H-2 reaction rate and 2-fold for the CO/H-2 reaction rate for the Co/TiO2 treated at 250 degrees C in H-2 versus Co/TiO2 treated at 450 degrees C. Inversely, the activity of cobalt supported on SiO2 has a higher turnover frequency when cobalt is metallic. The product distributions could be tuned depending on the support and the oxidation state of cobalt. For oxidized cobalt on TiO2, we observed an increase of methane production for the CO2/H-2 reaction whereas it is more selective to unsaturated products for the CO/H-2 reaction. In situ investigation of the catalysts indicated wetting of the TiO2 support by Cog, and partial encapsulation of metallic Co by TiO2-x.