The adsorption of soil organic matter (SOM) to minerals is considered to be a major mechanism protecting organic matter from biodegradation. In addition, the adsorption of organic matter on mineral surfaces influences the distribution and bioavailability of a range of nutrients and pollutants. However, when it comes to the nature of SOM in intimate association with mineral surfaces, there is no clear insight into the type and properties of that SOM fraction. This knowledge gap can be attributed to the complexity and heterogeneity of SOM itself and lack of understanding of the mechanisms of fractionation of SOM upon adsorption in a multi-component soil system. In this research, carefully designed lab experiments will be carried out to study the adsorption behavior of natural organic matter to goethite (to represent soil oxides). Advanced surface complexation modelling will be developed and applied to quantify and predict the complex reactions in the SOM-goethite system. The adsorption mechanisms and SOM fractionation revealed in the SOM-goethite system will form the basis to interpret characteristics of the heavy (organo-mineral associations) fractions of SOM which are physically fractionated based on density. The overall objective of this research is 1) to obtain a quantitative understanding of the mechanisms regulating fractionation of organic matter upon adsorption to oxides in soils, and 2) to obtain a better understanding of the type and properties of SOM associated with soil minerals, 3) to compare the results of 1) and 2) and to identify the similarities and differences between oxides and soil minerals in their preferences for SOM.