Natural Organic Matter (NOM) in soil and sediment interacts with mineral surfaces, primarily aluminosilicate or clay minerals, to form an organo-mineral composite known as humin. Humin typically represents more than 50% of the organic carbon present in soils and sediments. Like the other humic fractions of soil or sediment NOM, humin is defined operationally; it is defined as the humic fraction insoluble in an aqueous solution at any pH value. It is recognized as a primary sorbent for many Anthropogenic Organic Compounds (AOC) introduced into the solid or sediment environment. It also occupies an unique niche in the global geochemical cycle of carbon. Humin was isolated from four soil and sediment samples. Surface area, surface charge, porosity measurements and fractal analysis of small-angle x-ray scattering data were used to characterized surfaces properties as a result of selective removal of the various organic materials making up the composite using solvent extraction, the MIBK partitioning method, and bromine oxidation. The surface fractal dimension decreased while surface area increased, and surface pore size decreased upon organic matter removal. These results suggest that the mineral components of humin have smooth surfaces over length scales of ~1 to 15 nm, and that the organic coatings are responsible for surface roughness. The surface of humin particles were found to be dominated by micro- and mesopores that could be at least partly responsible for humin's high sorptive capacity for AOC.