The adsorption behavior of four perfluoroalkyl acids, including the environmentally relevant perfluorooctanoic acid, has been investigated on a straight-chain perfluorohexyl adsorbent. The aim of this study was to probe the potential of perfluorinated materials for the analysis and enrichment of perfluoroalkyl analytes. Water/acetonitrile mixtures, to which formic acid had been added (generally, 0.1%), were employed as mobile phases. For all perfluorinated acids, a U-shaped retention profile was observed by changing the amount of acetonitrile in the mobile phase. This behavior has been correlated to the excess adsorption of the organic component on the adsorbent. surface. The concept of perfluoromethylene selectivity has been defined as the ability of a chromatographic system to discriminate between molecules that differ by a single perfluoromethylene group. The contribution to the Gibbs free energy of phase transfer for the passage of a perfluoroalkyl carbon from the mobile to the stationary phase has been evaluated. This information, in addition to the traditional van't Hoff analysis, has also been used to estimate the analogous contribution for the transfer of a carboxylic unit. Finally, insights into the retention mechanisms of perfluoroalkyl acids on straight-chain perfluorohexyl sorbents are discussed.