A method for purifying oligonucleotides by displacement chromatography on anion-exchange media, using high affinity, low molecular weight (less than about 10000 Da) displacers, is disclosed. Several examples of high affinity, low molecular weight anionic displacers are provided, including polycyclic aromatic compounds having sulfonic acid moieties attached thereon. The efficacy of the technique for high resolution separation of oligonucleotides is demonstrated for an industrial mixture.