In this study, we apply the approach of Ahangar et al. (2008b) of comparing KOC for diuron before and after HF-treatment to a much larger and more diverse range of 34 soils selected to represent the diversity of soil and land-use systems in Sri Lanka. We have shown previously that diuron KOC for this set of whole soils varies widely ( Liyanage et al., 2006). Our aim here is to test the influence of organic matter–mineral interactions (by comparing KOC between corresponding whole and HF-treated soils) and the influence of the nature of organic matter (by linear regression of KOC against NMR-derived measures of organic matter chemistry) across this diverse range of soils.