Porous microparticulate zirconium oxide shows very different selectlvttles and pH dependencies for the separation of benzoic acid derivatives than do conventional bonded-phase anion-exchange supports. This results from a very significant ligand-exchange contribution to the retention of hard Lewis bases on the surface of transition-metal oxide supports. We have found that the capacity factors of a wide variety of derivatives of benzoic acid are closely correlated with their Bronsted acidities. The eluent pH Is also a critical factor In determining the magnitude of the capacity factor, but It does not have much Influence on chromatographic selectivity. The differential selectivity of this phase In comparison to conventional polymeric and bonded-phase anion exchangers can be attributed to complexatlon and steric effects which profoundly alter the elution patterns of certain solutes.