Chiral crystalline compounds of the series of [Ln(α2-P2W17O61)2]17- polyanions embracing nearly all the lanthanide (Ln) ions were successfully isolated using Pro (proline) as a chiral director. In each compound, the polyanion and Pro were associated through a stereoselective interaction involving N(H)···O hydrogen bonding. 31P NMR and circular dichroism studies revealed the induction of chirality by Pro in the racemic aqueous systems of the [Ln(α2-P2W17O61)2]17- polyanions, i.e., the Pfeiffer effect. Mechanistically, this racemization was regulated by competitive enantiomeric interactions between the l-/d-Pro and l-/d-polyanions, with only one racemization route (the on-site rotation of the [α2-P2W17O61)]10- unit) observed, independent of the nature of Ln.