As part of a programme of research into natural attenuation of contaminants in landfill liners, the sorption and desorption of Mecoprop (MCPP) by mineral liner materials was investigated. Mercia Mudstone and Oxford Clay have the potential to attenuate MCPP by sorption processes, which were observed to be rapid in both materials. Sorption to iron hydroxides was thought to be the main mechanism operating in Mercia Mudstone and may be responsible for some of the sorption occurring in Oxford Clay. Additional sorption to Oxford Clay may be related to its kerogen content, as this type of organic matter has been shown to be highly sorbent for hydrophobic organic compounds, but further research is needed to assess whether kerogen has a high affinity for polar compounds such as MCPP. Desorption tests demonstrated that desorption was rapid and that sorption was potentially reversible. There was some evidence that tannic acid in the synthetic leachate did not affect MCPP sorption, but the high ionic strength leachate reduced sorption of MCPP in Oxford Clay. For regulatory risk assessments, this result emphasizes the importance of conducting sorption tests in a matrix similar to that of the environment under investigation to obtain realistic results for sorption coefficients.