Enantiopure crystals of chiral [L-/D-Er(alpha(2)-P2W17O61)(2)](17-) polyanions were successfully resolved by using L-/D-proline (pro) as chiral auxiliaries. In both crystals, the chirality was stabilized by enantioselective interaction involving N-H center dot center dot center dot O hydrogen bonding between the polyanion and pro molecules. Circular dichroic spectroscopy in aqueous solutions revealed that partial racemization of the polyanions and the racemic equilibrium can be enantioselectively controlled by the amount of coexisting pro.