This study provides an evaluation of the sorption capacity of two contrasting mineral liners or barriers widely used in the UK for a range of organic contaminants of varying hydrophobicity commonly found in landfill leachate. Batch tests (involving toluene, trichlorobenzene, trichloroethene and naphthalene) showed that the sorption capacity of Oxford Clay was not only significantly greater than that of Mercia Mudstone, but was also greater than the sorption capacity of many soils or clays reported in the literature. The organic carbon normalized sorption coefficients (Koc) for Mercia Mudstone were comparable with both published and empirically derived Koc values, but the Koc for Oxford Clay was underestimated by literature values by several orders of magnitude. Retardation of these contaminants by Oxford Clay was also under-predicted by estimates based solely on organic carbon content. Amorphous organic matter (the main component of the organic matter in the Oxford Clay as characterized using ‘coal petrography’ methods) was believed to be responsible for the elevated sorption capacity of the Oxford Clay liner. Sorption coefficients were reduced in the presence of dissolved organic carbon in leachate, suggesting that published Koc values derived in synthetic groundwater may overestimate the sorption capacity in landfill scenarios. Sorption coefficients and Koc–Kow correlations determined in this study can be used for modelling organic contaminant sorption in Oxford Clay and Mercia Mudstone liners as part of landfill risk assessments in the absence of site-specific data, in particular for Oxford Clay, for which published correlations were shown to be too conservative. For other types of clay liner material, the cautious approach would be to determine site-specific sorption coefficients following characterization of the organic carbon. Further research is needed into the effects of leachate dissolved organic carbon and the composition of clay liner organic carbon on sorption of hydrophobic organic compounds.