A series of Mg-Al hydrotalcites (HTs) with lateral platelet sizes ranging from 40 nm to 2 mu m were prepared hydrothermally. Small HT platelets (similar to 20 nm) were obtained by deposition onto a carbon nanofiber (CNF) support. The CO2 sorption capacity at 523 K for the activated unsupported HT was low (similar to 0.1 mmol.g(-1)), regardless of the platelet size of the HT precursor. In addition, no relation was found between the CO2 sorption properties of activated HTs and their specific Surface area. The sorption capacity of the activated CNF-supported HTs was increased, depending on the HT loading, by an order of magnitude (1.3-2.5 mmol/g(HT)(-1)) compared to that of the activated unsupported HT. We propose that the CO2 sorption capacities of HTs are determined by the amounts of low-coordination oxygen sites in the Mg(Al)O-x nanoparticles, which is highest on supported HTs.