The detailed stoichiometry of the reaction between the aldopentoses, ribose, arabinose, lyxose, and xylose, and the cis rhodium(vertical bar vertical bar vertical bar) complex of the macrocyclic tetraamine ligand rac-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane in weakly acidic aqueous solution has been studied by C-13 NMR, using deuterium- and carbon-13-labeled substrates. The overall process is a catalytic disproportionation reaction, in which two aldopentose molecules are transformed into the corresponding alditol and alclonolactone, both with an unchanged configuration around carbon atoms 2, 3, and 4. The mechanism of this reaction is suggested to involve coordination of a hydrated and an unhydrated substrate molecule through their carbon-1 bound oxygen atoms followed by a hydride shift from the hydrated to the unhydrated substrate. This disproportionation process is subsequently followed by aquation of the reaction product to give the free alditol and a mixture of the aldonic acid and the corresponding alclonolactone. Concurrently with the aquation reaction, incorporation of solvent hydrogen at the carbon-1 atom of the alditol is also observed for the rhodium-coordinated alditol reaction product.