Nb(OR)(4)[OC(O)OR] (R = Me, Et, allyl) compounds have been synthesized by the reaction of dimeric alkoxo complexes [Nb(OR)(5)](2) with CO2. The kinetics of the reaction has shown that the monomer in equilibrium with the dimer is the species that reacts with CO2. The resulting compounds are monomeric with a eta(2)-O, O-bonded hemicarbonate group. The CO2 uptake is reversible. The hemicarbonates react with the parent alcohol to afford the relevant organic carbonate (RO)(2)CO with a turnover number (TON) of >2.